Avirulent microbes and uses therefor: Salmonella typhi

ABSTRACT

This invention provides immunogenic compositions for the immunization of a vertebrate or invertebrate comprising an avirulent derivative of S. typhi. The derivatives having a mutation of the cya and/or crp and/or cdt genes. The invention also provides immunogenic compositions for the immunization of a vertebrate and invertebrate comprising an avirulent derivative of the above type which is capable of expressing a recombinant gene derived from a pathogen of said vertebrate or invertebrate individual to produce an antigen capable of inducing an immune response against said pathogen. Other embodiments of the invention include methods of preparing immunogenic compositions from these strains, and strains useful in the preparation of the immunogenic compositions, as well as methods of stimulating the immune system to respond to an immunogenic antigen of S. typhi by administration of the immunogenic composition.

REFERENCE TO GOVERNMENT GRANT

This invention was made with government support under Grant Numbers AI 26186, DE 06669, and AI 24533. The government has certain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 07/975,892, filed Nov. 13, 1992 now abandoned, which is a continuation of U.S. application Ser. No. 07/612,001, filed Nov. 9, 1990 now abandoned, which is a continuation-in-part of copending U.S. application Ser. No. 200,934, filed Jun. 1, 1988, now abandoned in favor of continuation application Ser. No. 07/965,607 filed Oct. 22, 1992 now abandoned, which is a continuation-in-part of copending U.S. application Ser. No. 058,360, filed Jun. 4, 1987, now abandoned; it is also a continuation-in-part of copending U.S. application Ser. No. 251,304, filed Oct. 3, 1988, now abandoned in favor of continuation application Ser. No. 07/990,361 filed Dec. 15, 1992, now abandoned which is a continuation-in-part of copending U.S. application Ser. No. 106,072, filed Oct. 7, 1987, now abandoned. These applications are hereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to avirulent microbes, their method of preparation, and their use in vaccines.

BACKGROUND OF THE INVENTION

Typhoid fever, which is caused by Salmonella typhi, remains an important public health problem for residents in the less developed world, for travelers from industrialized countries who visit endemic areas, and for clinical microbiologists in laboratories which conduct proficiency tests. The currently licensed parenteral killed whole cell typhoid vaccines are protective but cause marked systemic and local adverse reactions at an unacceptably high frequency (Levine, Typhoid fever vaccines, in Plotkin SA, Mortimer EA Jr. (eds): VACCINES. Philadelphia, WB Saunders, 1988, pp. 333-361). Alternative vaccines include the recently licensed live oral vaccine strain Ty21a and the experimental parenteral Vi polysaccharide vaccine.

The advantage of an oral vaccine is the delivery of replicating organisms to the mucosal immune system where local responses are maximally stimulated. In addition, attenuated Salmonella typhi are attractive candidates to serve as carrier vaccines to express foreign antigens and deliver them to the human immune system. However, a critical prerequisite for successfully using this approach in immunizing humans is that there must exist highly immunogenic yet salfe attenuated strains of S. typhi to deliver the foreign protein and polysaccharide antigens to the immune system.

The current oral vaccine based upon Ty21a has several disadvantages. Ty21a is of relatively low immunogenicity and requires multiple oral doses to immunize. The yield of viable organisms is low when it is fermented and lyophilized in large-scale. In addition, Ty21a has multiple mutations in addition to galE and via, which remain undefined. (Hone et al. (1987), J. Infect. Dis. 156:167-174; Hone et al. (1988), J. Infect. Immun. 56:1326-1333).

Constructs of Ty21a expressing the O antigen of Shigella sonnei (Formal et al (1981), Infect. Immun. 34:746-750) or the O antigen of Vibrio cholerae 01 serotype Inaba (Forrest et al. (1989), J. Infect. Dis. 159:145-146) have undergone clinical testing in humans. Although two lots of the Ty21a/S. sonnei construct tested in North American volunteers provided significant protection against experimental challenge with pathogenic S. sonnei, there was lot-to-lot variation and other lots were not protective (Black et al. J. Infect. Dis. (1987), 155:1260-1627; Herrington et al. (1990), Vaccine 8:353-357). The Ty21a/Inaba construct elicited serum Inaba vibriocidal antibodies and intestinal SIgA anti-Inaba O antibodies in only a minority of vaccinees and at low titer (Tacket et al. (1990), Infect. Immun. 58:1620-1627). In experimental challenge studies with pathogenic V. cholerae 01, recipients of the construct were not significantly protected overall against diarrhea, but did have milder diarrhea and shed fewer wild-type V. cholerae cells (Tacket et al., Id.).

The main drawbacks to the use of Ty21a as a candidate carrier strain include its limited immunogenicity, a lack of precise information on the molecular basis of its attenuation and practical difficulties in bacterial genetic manipulation of the strain (e.g., in transformation, electroporation, and recombination frequency). It also exhibits very poor viability after reconstitution of lyophilized cultures.

Applicant has discovered new methods of protecting against virulent infections with vaccines employing transposon-induced avirulent mutants of virulent agents in which the impairment leading to avirulence cannot be repaired by diet or by anything supplied by an animal host. Some of Applicant's initial work, including a method for creating an avirulent microbe by the introduction of deletion mutations in the adenylate cyclase gene (cya) and the cyclic AMP receptor protein gene (crp) of Salmonella typhimurium is described in EPO Pub. No. 315,682 (published May 17, 1989), and PCT Pub. No. WO 88/09669 (published Dec. 15, 1988). Applicant has also provided methods for producing other types of avirulent mutant cells which are desirable as carrier cells for the expression of recombinant antigens. These cells are characterized by a lack of a functioning native gene encoding an enzyme which is essential for cell survival, wherein the enzyme catalyses a step in the biosynthesis of an essential cell wall structural component and the presence of a first recombinant gene encoding an enzyme which is a functional replacement for the native enzyme, wherein the first recombinant gene cannot replace the defective chromosomal gene. In these cells, the first recombinant gene is structurally linked to a second recombinant gene encoding a desired product. Loss of the first recombinant gene causes the cells to lyse. These methods are described in WO 89/03427 (published Apr. 20, 1989). The disclosures of the above-described patent applications, as well as any corresponding national patent applications, are incorporated herein by reference.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is based, in part, on new avirulent S. typhi derivatives that are not disclosed in EPO Pub. No. 315,682. Included within the invention is the application of these new S. typhi derivatives in, inter alia, commercial vaccines, methods of stimulating the immune system to respond to an immunogenic antigen of S. typhi, and methods of stimulating the immune system to respond to an immunogenic antigen of a pathogen. The strains provided herein are directly and indirectly suitable for the production of commercial vaccines to prevent diseases caused by S. typhi, and other enteric bacterial with which antibodies to S. typhi cross react. These strains are also useful as carrier microorganisms for the production of expression products encoded on recombinant genes in the bacterial cells.

Accordingly, one embodiment of the invention is an immunogenic composition for the immunization of an individual comprising an avirulent derivative of Salmonella typhi (S. typhi) with a mutation in a cya gene, in a physiologically acceptable excipient.

Another embodiment of the invention is an isolated avirulent strain of S. typhi with a mutation in a cya gene.

Yet another embodiment of the invention is an immunogenic composition for the immunization of an individual comprising an avirulent derivative of S. typhi with a mutation in a crp gene.

Still another embodiment of the invention is an isolated avirulent strain of S. typhi with a mutation in a crp gene.

Another embodiment of the invention is an immunogenic composition for the immunization of an individual comprising an avirulent derivative of S. typhi, said derivative with a mutation in a cya gene and a mutation in a crp gene.

Yet another embodiment of the invention is an isolated avirulent strain of S. typhi with a mutation in a cya gene and a crp gene.

Still another embodiment of the invention is an immunogenic composition for the immunization of an individual comprising an avirulent derivative of Salmonella with a mutation in a cdt gene.

Another embodiment of the invention is an isolated avirulent strain of Salmonella which contains a mutation in a cdt gene.

Still another embodiment of the invention is an isolated strain selected from the group of strains .sub.χ 3958, .sub.χ 4323, .sub.χ 3926, .sub.χ 3927, .sub.χ 4297, .sub.χ 4346, .sub.χ 3940, .sub.χ 4073, ISP2822, and ISP1820, and derivatives thereof.

Another embodiment of the invention is a method of utilizing a strain of avirulent S. typhi which is comprised of a mutation in a cya gene, the method comprising preparing an immunogenic composition by suspending the strain in a physiologically acceptable excipient.

Yet another embodiment of the invention is a method of utilizing a strain of avirulent S. typhi which is comprised of a mutation in a crp gene, the method comprising preparing an immunogenic composition by suspending the strain in a physiologically acceptable excipient.

Still another embodiment of the invention is a method of utilizing a strain of avirulent Salmonella which is comprised of a mutation in a cdt gene, the method comprising preparing an immunogenic composition by suspending the strain in a physiologically acceptable excipient.

Another embodiment of the invention is an isolated strain of S. typhi which is a cdt mutant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1.A. is a graph which shows the recovery of CFU from the Peyer's patches of 8 week old BALB/c mice at specified times after peroral inoculation with 9×10⁸ CFU of .sub.χ 3622 (Δ[crp-cysG]-10), 1×10⁹ CFU of .sub.χ 3737 (pSD110⁺ /Δ[crp-cysG]-10) and 1×10⁹ CFU of .sub.χ 3339 (wild type). Three mice were sacrificed for each time point. The results are given as geometric means ± standard deviations.

FIG. 1.B. is a graph which shows the recovery of CFU from the spleens of 8-week-old BALB/c female mice at specified times after peroral inoculation with 9×10⁸ CFU of .sub.χ 3622 (Δ[crp-cysG]-10), 1×10⁹ CFU of .sub.χ 3737 (pSD110⁺ /Δ[crp-cysG]-10) and 1×10⁹ CFU of .sub.χ 3339 (wild type). Three mice were sacrificed for each time point. The results are given as geometric means ± standard deviations.

FIG. 2 is a partial restriction map of pYA1077. The 1.0 Kb M. leprae insert DNA fragment from λgt11 clone L14 was subcloned into the EcoRI site of pYA292. There is a single asymmetrical SalI site within the M. leprae insert DNA. There are no sites within the M. leprae insert DNA for the following restriction endonucleases: BamHI, HindIII, PstI, and XbaI.

FIG. 3 is a half-tone reproduction showing a Western blot of proteins produced by S. typhi, S. typhimurium, and E. coli strains harboring pYA1077 and pYA1078. The proteins on the nitrocellulose filters were reacted with pooled sera from 21 leptromatous leprosy patients. Lane 1, molecular size markers (sizes are indicated to the left of the blot); Lane 2, proteins specified by S. typhi .sub.χ 4297 with pYA292; Lanes 3 to 5, proteins specified by three independent S. typhi .sub.χ 4297 isolates each containing pYA1078; Lanes 6 to 8, proteins specified by three independent isolates of S. typhi .sub.χ 4297 isolates each containing pYA1077; Lane 9, proteins specified by S. typhimurium .sub.χ 4072 with pYA1077; Lane 10, proteins specified by E. coli .sub.χ 6060 with pYA1075 (a pUC8-2 derivative containing the 1.0 kb M. leprae DNA insert from λgt11 clone L14 in the same orientation relative to the lacZ promoter as it is in pYA1077). Note: the immunologically reactive protein specified by pYA1075 is slightly larger than that specified by pYA1077 because it is a fusion protein with the alpha region of β-galactosidase.

MODES FOR CARRYING OUT THE INVENTION

This invention is predicated on the discovery that certain mutations can render a microbe avirulent without substantially affecting its immunogenicity. More specifically, this invention relates to microbial vaccines in which the microbe carries the deletion (open triangle or delta) mutations Δcya and/or Δcrp eliminating the ability to synthesize adenylate cyclase (ATP pyrophosphate lyase (cyclizing) EC 4.6.1.1) and the cyclic AMP receptor protein (CRP), respectively.

Cyclic-3'5'-AMP (cAMP) and the cyclic AMP receptor protein are necessary for the transcription of a large number of genes and operons concerned with the transport and breakdown of a large number of catabolites. Evidence has been provided that shows that systems used for transporting fuel/carbon sources are all under positive control by cAMP, as are several amino acid permeases. In addition to its very important role in catabolism, the cAMP concentration in cells also influences lysogenization by temperate phages, synthesis of fimbriae, synthesis of flagella and synthesis of at least one outer membrane protein. Although cAMP is present in mammalian cells, the concentrations present in macrophages and other cells in which Salmonella can invade and multiply are below the concentration of 0.1 to 1.0 mM cAMP necessary to allow Δcya mutants to exhibit a wild-type phenotype in vitro. Furthermore, the inclusion of the Δcrp mutation would essentially abolish any benefit that could accrue from uptake of cAMP in vitro or in vivo by such Δcya mutants.

Introduction of the mutations into cya and crp of S. typhi can be accomplished by use of transposons, to transfer the mutations from other Salmonella strains into S. typhi. Transposons can be added to a bacterial chromosome at many points. The characteristics of transposon insertion and deletion have been reviewed in Kleckner et al. (1977), J. Mol. Biol. 116:125. For example, the transposon Tn10, which confers resistance to tetracycline (and sensitivity to fusaric acid) can be used to create Δcya and Δcrp mutations in a variety of bacterial species, including, for example, E. coli and S. typhimurium. Methods for the creation and detection of these mutants in S. typhimurium are described in EPO Pub. No. 315,682, and a method is also provided in the Examples, infra. Utilizing Tn10, these mutations can be transposed into various isolates of S. typhi, preferably those which are highly pathogenic. Examples of the transfer of the Δcya and Δcrp mutations from S. typhimurium into wild type S. typhi strains are shown in the Examples, infra.

Once rendered avirulent by the introduction of the Δcya and/or Δcrp mutations, the microbes can serve as the immunogenic component of a vaccine to induce immunity against the microbe.

In another embodiment of the invention, the S. typhi which are cya mutants and/or crp mutants are further mutated, preferably by a deletion, in a gene adjacent to the crp gene which governs virulence of Salmonella. Mutation in this gene, the cdt gene, diminishes the ability of the bacteria to effectively colonize deep tissues, e.g., the spleen. When a plasmid having the crp⁺ gene is placed in a strain with the Δ(crp-cdt), it retains its avirulence and immunogenicity thus having a phenotype similar to cya and crp mutants. Mutants with the Δ(crp-cdt) mutation containing a crp⁺ gene on a plasmid retain the normal ability to colonize the intestinal tract and GALT, but have a diminished ability to colonize deeper tissues. In the Examples, the original Δ(crp-cdt) mutation as isolated in .sub.χ 3622 also deleted the argD and cysG genes imposing requirements for arginine and cysteine for growth; this mutant allele has been named Δ(crp-cysG)-10. A second mutant containing a shorter deletion was isolated that did not impose an arginine requirement; it is present in .sub.χ 3931 and has been named Δ(crp-cysG)-14. Mutations in cdt in S. typhi can be either created directly, or can be introduced via transposition from S. typhimurium strains such as those shown in the Examples. In addition, the cdt mutation can be created in other strains of Salmonella using techniques known in the art, and phenotypic selection using the characteristics described herein; alternatively, the Δcdt mutation can be transposed from the S. typhimurium described in the Examples into other strains of Salmonella using techniques of transposon mutagenesis which are known in the art.

In still another embodiment of the invention, the avirulent derivative of a pathogenic S. typhi can be used as a carrier bacteria to deliver selected antigens to the GALT, for example to the Peyer's patches of the ileum. Salmonella are known to home to the Peyer's patches (Carter, P. B. and F. M. Collins, J. Exp. Med. 139.:1189 (1974)). S. typhimurium-E. coli hybrids have also been shown to colonize Peyer's patches in mice (Hohmann, A. W., et al., Infect. and immun. 22:763 (1978)). If these carrier bacteria contain and express a recombinant gene from a pathogenic organism, antibodies against the antigenic gene product produced from the pathogen will be induced. With the advent of recombinant DNA techniques, it now becomes possible to develop totally unique vaccines in which specific antigens are produced, not by the etiologic agent, but by another host strain of bacteria capable of expressing the gene for that antigen. It is also possible, when antigens might cross-react with an antigen of the mammalian host and thus potentiate the induction of autoimmunity, to use recombinant DNA techniques to alter the gene so that the affecting cross-reacting antigenic determinant is not produced. Thus, recombinant DNA techniques can be employed to develop vaccines that do not have any material capable of cross-reacting with vertebrate host antigens or capable to eliciting an autoimmune state.

Methods of preparing organisms, particularly Salmonella, which can function as carrier bacteria are discussed in WO 89/03427 (published Apr. 20, 1989), and in U.S. Ser. No. 07/251,304, filed Oct. 3, 1988, which is commonly owned. Both of these references are incorporated herein by reference. Generally, the Salmonella are treated to cause a mutation in a chromosomal gene which encodes an enzyme that is essential for cell survival, wherein this enzyme catalyzes a step in the biosynthesis of an essential cell wall structural component. An extrachromosomal genetic element, for example, a recombinant vector, is introduced into the mutant cell. This genetic element contains a first recombinant gene which encodes an enzyme which is a functional replacement for the native enzyme, but the first recombinant gene cannot replace the defective chromosomal gene. The first recombinant gene is structurally linked to a second recombinant gene encoding a desired product, which is to be expressed in the carrier microorganism. Loss of the first recombinant gene causes the cells to lyse when the cells are in an environment where a product due to the expression of the first recombinant gene is absent.

A number of genes which encode enzymes essential for cell survival, which catalyze a step in the biosynthesis of an essential cell wall structural component, are known in the art, for e.g., aspartate semialdehyde dehydrogenase (Asd), which is encoded by the asd gene. A method for introducing a deletion mutation in the asd gene of S. typhi utilizing transposon mutagenesis is shown in the Examples. Also shown in the Examples, is the construction of a genetic element which carries the functional replacement for the asd gene, linked to a gene encoding an antigen which is to be expressed in the avirulent carrier S. typhi.

It is apparent that the present invention has wide applicability to the development of effective vaccines against bacterial, fungal, parasite or viral disease agents where local immunity is important and might be a first line of defense. Some examples are vaccines for the control of pneumonic plague caused by Yersinia pestis, of gonorrhea caused by Neisseria gonorrhoeae, of syphilis caused by Treponema pallidum, and of venereal diseases as well as eye infections caused by Chlamydia trachomatis. Species of Streptococci from both group A and group B, such as those species that cause sore throat or heart disease, Neisseria meningitidis, Mycoplasma pneumoniae, Haemophilus influenzae, Bordetella pertussis, Mycobacterium tuberculosis, Mycobacterium leprae, Bordetella avium, Escherichia coli, Streptococcus equi, Streptococcus pneumoniae, Brucella abortus, Pasteurella hemolytica, Vibrio cholerae, Shigella species, and Legionella pneumophila are additional examples of bacteria within the scope of this invention from which genes could be obtained. Viral vaccines, such as those produced against influenza viruses, are also encompassed by this invention. Viral vaccines can also be produced against other viruses, either DNA or RNA viruses, for example from the classes Papovirus, Adenovirus, Herpesvirus, Poxvirus, Parvovirus, Reovirus, Picornavirus, Myxovirus, Paramyxovirus, or Retrovirus. Vaccines to protect against infection by pathogenic fungi, protozoa and parasites are also contemplated by this invention.

In a further embodiment, when the immunogenic component of the vaccine is an allergen of the host such a vaccine may be used in an exposure regimen designed to specifically desensitize an allergic host.

In one of its embodiments, the invention can be described as a vaccine for the immunization of a vertebrate animal or invertebrate comprising a live avirulent derivative of a pathogenic microbe said derivative being incapable of producing functional adenylate cyclase and cAMP receptor protein while being capable of expressing a recombinant gene derived from an organism that is a pathogen of or that produces an allergen of said animal.

In yet another embodiment the avirulent microbes of this invention may be used as vectors for the synthesis of various host proteins. Because the avirulent microbes of this invention are able to traverse a variety of immunocompetent structures including GALT, mesenteric lymph nodes and spleen after introduction into the host, such microbes may be used to target a variety of immunoregulatory products. Accordingly, one or more genes encoding immunoregulatory proteins or peptides may be recombinantly introduced into the avirulent microbes such that when the microbes taking up residence in the appropriate immunocompetent tissue are capable of expressing the recombinant product to suppress, augment or modify the immune response in the host. Examples of immunoregulatory molecules include but are not limited to: colony stimulating factors (macrophage, granulocyte, or mixed), macrophage chemotoxin, macrophage inhibition factor, leukocyte inhibitory factors, lymphotoxins, blastogenic factor, interferon, and interleukins.

Still another embodiment of the subject invention is the use of the avirulent microbes contemplated herein to deliver and produce pharmacologically active products that might stimulate or suppress various physiological functions (i.e., growth rate, blood pressure, etc.).

In an embodiment which contemplates all of the above, a subject of the invention is avirulent strains of S. typhi, which carry mutations in the cya and/or crp genes.

The creation of cya and/or crp mutants of S. typhi, including those which also contain mutations in cdt, and/or the mutations in carrier microorganisms can also be accomplished using other techniques known in the art. These techniques include for example, standard techniques of mutagenesis and/or the use of recombinant DNA techniques. The desired mutants are then selected on the basis of phenotypic characteristics, some of which are described infra., in the Examples, and which are shown in Table 8. Each of the terms in these embodiments of the invention is analyzed in the following discussion.

By immunogenic agent is meant an agent used to stimulate the immune system of a living organism, so that one or more functions of the immune system are increased and directed towards the immunogenic agent. Immunogenic agents include vaccines. Immunogenic agents can be used in the production of antibodies, both isolated polyclonal antibodies and monoclonal antibodies, using techniques known in the art.

By vaccine is meant an agent used to stimulate the immune system of a living organism so that protection against future harm is provided. Immunization refers to the process of inducing a continuing high level of antibody and/or cellular immune response in which T-lymphocytes can either kill the pathogen and/or activate other cells (e.g., phagocytes) to do so in an organism, which is directed against a pathogen or antigen to which the organism has been previously exposed. Although the phrase "immune system" can encompass responses of unicellular organisms to the presence of foreign bodies, e.g., interferon production, in this application the phrase is restricted to the anatomical features and mechanisms by which a multi-cellular organism produces antibodies against an antigenic material which invades the cells of the organism or the extra-cellular fluid of the organism. The antibody so produced may belong to any of the immunological classes, such as immunoglobulins A, D, E, G or M. Of particular interest are vaccines which stimulate production of immunoglobulin A (IgA) since this is the principle immunoglobulin produced by the secretory system of warm-blooded animals, although vaccines of the invention are not limited to those which stimulate IgA production. For example, vaccines of the nature described herein are likely to produce a broad range of other immune responses in addition to IgA formation, for example, cellular and humoral immunity. Immune response to antigens is well studied and widely reported. A survey of immunology is given in Barrett, James, T., Textbook of Immunology: Fourth Edition, C. V. Mosby Co., St. Louis, Mo. (1983), the entire of which is herein incorporated by reference.

A vertebrate is any member of the subphylum Vertebrata, a primary division of the phylum Chordata that includes the fishes, amphibians, reptiles, birds, and mammals, all of which are characterized by a segmented bony or cartilaginous spinal column. All vertebrates have a functional immune system and respond to antigens by producing antibodies. Thus, all vertebrates are capable of responding to vaccines. Although vaccines are most commonly given to mammals, such as humans or dogs (rabies vaccine), vaccines for commercially raised vertebrates of other classes, such as the fishes and birds if of the nature described herein, are within the scope of the present invention.

An invertebrate is any member of the Animal Kingdom, excluding the vertebrates. Such animals constitute the Division Invertebrata and have no backbone or spinal column. This classification includes all animals except fishes, amphibians, reptiles, birds and mammals. Many invertebrates are capable of illiciting a primitive immune response to antigenic stimulation and are susceptible to the same microorganisms which infect vertebrates and which are disclosed herein in accordance with this invention. Exemplary of such invertebrates are shellfish and molluses and other related animals. Although the use of vaccines in the protection of invertebrate animals have hitherto before not been well documented, one skilled in the art will recognize the applicability of the subject invention to said invertebrates by use of their primitive immune systems. For example, and in accordance with this invention, the susceptibility of shellfish to infection by Salmonella will allow the introduction of avirulent strains of Salmonella species and thereby provide potential for the primitive immune system to respond. Therefore, it is within the scope of this invention, the use of an avirulent derivative of a pathogenic microbe, that is capable of infecting an invertebrate, to stimulate a response from an immune system present in said invertebrate against a pathogen.

An "individual" treated with a vaccine of the invention is defined herein as including all vertebrates, for example, mammals, including domestic animals and humans, various species of birds, including domestic birds, particularly those of agricultural importance. In addition, mollusks and certain other invertebrates have a primitive immune system, and are included as an "individual".

In one embodiment of the invention is the use of an avirulent derivative of a pathogenic microbe that attaches to, invades and persits in the GALT or BALT as a carrier of the gene product which is used for stimulating antibody response against a pathogen or allergen. Avirulent does not mean that a microbe of that genus or species cannot ever function as a pathogen, but that the particular microbe being used is avirulent with respect to the particular animal being treated. The microbe may belong to a genus or even a species that is normally pathogenic but must belong to a strain that is avirulent. By pathogenic is meant capable of causing disease or impairing normal physiological functioning. Avirulent strains are incapable of inducing a full suite of symptoms of the disease that is normally associated with its virulent pathogenic counterpart. Microbes as used herein include bacteria, protozoa, and unicellular fungi.

Techniques for transferring genetic material from a first organism to a second organism which normally does not exchange genetic material with the first organism, have recently become widely available as the result of rapidly expanding recombinant DNA technology. In this application, genetic material that has been transferred from one organism into a second in such a manner that reproduction of the second organism gives rise to descendents containing the same genetic material is referred to as a recombinant gene. The term gene is being used here in its broadest sense to represent any biological unit of heredity. It is not necessary that the recombinant gene be a complete gene as present in the parent organism, which was capable of producing or regulating the production of a macromolecule, for example, a functioning polypeptide. It is only necessary that the gene be capable of serving as the template used as a guide in the production of an antigenic product. The product may be one that was not found in that exact form in the parent organism. For example, a functional gene coding for a polypeptide antigen comprising 100 amino acid residues may be transferred in part into a carrier microbe so that a peptide comprising only 75, or even 10, amino acid residues is produced by the cellular mechanism of the host cell. However, if this gene product is an antigen that will cause formation of antibodies against a similar antigen present in the parent organism, the gene is considered to be within the scope of the term gene as defined in the present invention. Alternatively, if the amino acid sequence of a particular antigen or fragment thereof is known, it is possible to chemically synthesize the DNA fragment or analog thereof by means of automated gene synthesizers or the like and introduce said DNA sequence into the appropriate expression vector. At the other end of the spectrum is a long section of DNA coding for several gene products, one or all of which can be antigenic. Thus, a gene as defined and claimed here is any unit of heredity capable of producing an antigen. The gene may be of chromosomal, plasmid, or viral origin.

In order for the gene to be effective in eliciting an immune response, the gene must be expressed. Expression of a gene means that the information inherent in the structure of the gene (the sequence of DNA bases) is transformed into a physical product in the form of an RNA molecule, polypeptide or other biological molecule by the biochemical mechanisms of the cell in which the gene is located. The biological molecule so produced is called the gene product. The term gene product as used here refers to any biological product or products produced as a result of the biochemical reactions that occur under the control of a gene. The gene product may be, for example, an RNA molecule, a peptide, or a product produced under the control of an enzyme or other molecule that is the initial product of the gene, i.e., a metabolic product. For example, a gene may first control the synthesis of an RNA molecule which is translated by the action of ribosomes into an enzyme which controls the formation of glycans in the environment external to the original cell in which the gene was found. The RNA molecule, the enzyme, and the glycan are all gene products as the term is used here. Any of these as well as many other types of gene products, such as glycoproteins and polysaccharides, will act as antigens if introduced into the immune system of an animal. Protein gene products, including glycoproteins and lipoproteins, are preferred gene products for use as antigens in vaccines.

In order for a vaccine to be effective in producing antibodies, the antigenic material must be released in such a way that the antibody-producing mechanism of the vaccinated animal can come into play. Therefore, the microbe carrier of the gene product must be introduced into the animal. In order to stimulate a preferred response of the GALT or BALT cells as discussed previously, introduction of the microbe or gene product directly into the gut or bronchus is preferred, such as by oral administration, gastric intubation or in the form of aerosols, although other methods of administering the vaccine, such as intravenous, intramuscular, subcutaneous injection or intramammary or intrapenial or vaginal administration, are possible.

When the avirulent microbe is used as a carrier microbe, and once the carrier microbe is present in the animal, the antigen needs to become available to the animal's immune system. This may be accomplished when the carrier microbe dies so that the antigen molecules are released. Of course, the use of "leaky" avirulent mutants that release the contents of the periplasm without lysis is also possible. Alternatively, a gene may be selected that controls the production of an antigen that will be made available by the carrier cell to the outside environment prior to the death of the cell. In this way, it is possible to use a viable microbe that will persist in the vaccinated animal, for example in its Peyer's patches, and continue to produce antigen, thereby continually inducing antibody formation. A preferred gene product under these circumstances is a product that is transferred through the cell membrane into the external environment or a product that becomes attached to or embedded in the external membrane so that all or part of the gene product is exposed to the environment. Typical of this latter type of gene product are antigens normally found on the surface of the organism against which protection is desired. If these antigens are transported to the cell surface in a normal manner, antibody formation against the antigens will be enhanced.

The use of pathogens to deliver antigens from other pathogens to the GALT or BALT would be inappropriate if it were not for the fact that such pathogens can be rendered avirulent while retaining ability to colonize Peyer's patches or the BALT.

The organism from which the recombinant gene is derived may be any pathogen of the animal being vaccinated or may be an organism that produced an allergen or other antigen of the animal. Allergens are substances that cause allergic reaction, in this case in the animal which will be vaccinated against them. Many different materials may be allergens, such as animal dander and pollen, and the allergic reaction of individual animals will vary for any particular allergen. It is possible to induce tolerance to an allergen in an animal that normally shows an allergic response. The methods of inducing tolerance are well-known and generally comprise administering the allergen to the animal in increasing dosages. Further discussion of tolerance induction is given in the Barrett textbook previously cited. Lastly, the host organism itself can serve as a source of genetic material when immunoregulatory genes or genes for other pharmacologically active substances are being expressed by the vectors.

Administration of a live vaccine of the type disclosed above to an animal may be by any known or standard technique. These include oral ingestion, gastric intubation, or broncho-nasal-ocular spraying. All of these methods allow the live vaccine to easily reach the GALT or BALT cells and induce antibody formation and are the preferred methods of administration. Other methods of administration, such as intravenous injection, that allow the carrier microbe to reach the animal's blood stream may be acceptable. Intravenous, intramuscular or intramammary injection are also acceptable with other embodiments of the invention, as is described later.

Since preferred methods of administration are oral ingestion, aerosol spray and gastric intubation, preferred carrier microbes are those that belong to species that attach to, invade and persist in any of the lymphoepithelial structures of the intestines or of the bronchii of the animal being vaccinated. These strains are preferred to be avirulent derivatives of enteropathogenic strains produced by genetic manipulation of enteropathogenic strains. Strains that attach to, invade and persist in Peyer's patches and thus directly stimulate production of IgA are most preferred. In animals these include specific strains of Salmonella, and Salmonella-E. coli hybrids that home to the Peyer's patches.

Recombinant DNA techniques are now sufficiently well known and widespread so as to be considered routine. In very general and broad terms, this method consists of transferring the genetic material, or more usually part of the genetic material, of one organism into a second organism so that the transferred genetic material becomes a part of the genetic material of the organisms to which it is transferred. This usually consists of first obtaining a small piece of DNA from the parent organism either from a plasmid or a parent chromosome. A plasmid (also called an extrachromosomal element) is a hereditary unit that is physically separate from the chromosome of the cell. The DNA may be of any size and is often obtained by the action of a restriction endonuclease enzyme which acts to split DNA molecules at specific basepair sites. Following ligation to plasmid, phage or cosmid vectors to form recombinant molecules the recombinant molecules may be transferred into a host cell by various means such as transformation (uptake of naked DNA from the external environment, which can be artificially induced by the presence of various chemical agents, such as calcium ions), including electroporation. Other methods such as transduction are also suitable, wherein the recombinant DNA is packaged within a phage such as transducing phage or cosmid vectors. Once the recombinant DNA is in the carrier cell, it may continue to exist as a separate piece (generally true of complete transmitted plasmids) or it may insert into the host cell chromosome and be reproduced with the chromosome during cell division.

Although transferring genetic material is relatively straightforward, predicting which transfers will result in expressed genes is not yet possible. This selection process, however, does not present any difficulty to the present invention. Since the host microbe must express the transferred gene and thereby produce an antigen, a "shotgun" approach works well. Antibodies are first produced against the desired antigen, for example, fragments of cell membranes from pathogenic microbes, by standard techniques. DNA from the organism that is the source of the antigen is cleaved into multiple fragments by endonucleases, and the fragments are inserted randomly into carrier microbes that express antigens from the pathogen can be easily identified by their reaction with antibody against pathogen antigens. Antigen-expressing microbes can be selected and cloned to give the desired recombinant organism. Shotgun cloning is well known and is described in detail in Maniatis, T., et al., Molecular Cloning, Second Edition, Cold Spring Harbor Laboratories (1989), which is herein incorporated by reference. The techniques of gene transfer are not considered to be part of this invention, and any method capable of producing recombinant organisms comprising genes from an organism that are expressed in avirulent microbes will suffice.

In cases where the species normally exchange genetic information more classical methods of gene transfer may be employed such as conjugation, transformation or transduction.

Derivatives of avirulent microbes are also contemplated to be within the scope of this invention. By derivative is meant sexually or asexually derived progeny and mutants of the avirulent strains including single or multiple base substitutions, deletions, insertions or inversions which retain the inability to produce functional adenylate cyclase and/or cAMP receptor protein and/or the expression of the cdt gene, with or without naturally occurring virulence plasmids. For example, strains such as 4062 and 4064 carry the gyrA mutation conferring nalidixic acid resistance which has been used herein as a convenient marker to follow strains through the animal following oral inoculation. However, drug resistance is not a desirable attribute for strains to be used as vaccines. Thus, the gyrA mutation can be easily removed by transducing the wild-type gyrA+ (conferring sensitivity to nalidixic acid) gene into strains by selecting for inheritance of a closely linked Tn10 and then removing Tn10 by transduction with a phage lysate propagated on the parent strain carrying the gyrA⁻ allele with selection for fusaric acid resistance.

The dosages required will vary with the antigenicity of the gene product and need only be an amount sufficient to induce an immune response typical of existing vaccines. Routine experimentation will easily establish the required amount. Multiple dosages are used as needed to provide the desired level of protection.

The pharmaceutical carrier or excipient in which the vaccine is suspended or dissolved may be any solvent or solid or encapsulated in a material that is non-toxic to the inoculated animal and compatible with the carrier organism or antigenic gene product. Suitable pharmaceutical carriers include liquid carriers, such as normal saline and other non-toxic salts at or near physiological concentrations, and solid carriers, such as talc or sucrose and which can also be incorporated into feed for farm animals. Adjuvants may be added to enhance the antigenicity if desired. When used for administering via the bronchial tubes, the vaccine is preferably presented in the form of an aerosol.

Immunization with a pathogen-derived gene product can also be used in conjunction with prior immunization with the avirulent derivative of a pathogenic microorganism acting as a carrier to express the gene product specified by a recombinant gene from a pathogen. Such parenteral immunization can serve as a booster to enhance expression of the secretory immune response once the secretory immune system to that pathogen-derived gene product has been primed by immunization with the carrier microbe expressing the pathogen-derived gene product to stimulate the lymphoid cells of the GALT or BALT. The enhanced response is known as a secondary, booster, or anamnestic response and results in prolonged immune protection of the host. Booster immunizations may be repeated numerous times with beneficial results.

The above disclosure generally describes the present invention. A more complete understanding can be obtained by reference to the following specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.

EXAMPLES Example 1

This example describes the isolation of avirulent microbes by the introduction of deletion mutations affecting cAMP synthesis and utilization and the identification of strains with mutations conferring stability of phenotype, complete avirulence and high immunogenicity.

Bacterial strains. The Escherichia coli and Salmonella typhimurium strains used are listed in Table 1.A. and B. They were maintained as frozen cultures suspended in 1% Bacto-peptone containing 5% glycerol and fast-frozen in dry ice-ethanol for storage in duplicate at -70° C. and also suspended in 1% Bacto-peptone containing 50% glycerol for storage at -20° C. for routine use.

Media. Complex media for routine cultivation were L broth (Lennox, Virology 1:190-206, (1965)) and Luria broth (Luria and Burrous, J. Bacteriol. 74:461-476 (1957)). Difco agar was added to Luria broth at 1.2% for base agar and 0.65% for soft agar. Penassay agar was used for routine enumeration of bacteria. Fermentation was evaluated by supplementing MacConkey base agar or Eosin methylene blue agar (Curtiss, Genetics 58:9-54 (1968)) with 1% final concentration of an appropriate carbohydrate.

Synthetic media were minimal liquid (ML) and minimal agar (MA) supplemented with nutrients at optimal levels as previously described (Curtiss, J. Bacteriol. 89:28-40, (1965)). Buffered saline with gelatin (BSG) (Curtiss, 1965 supra) was used routinely as a diluent.

Transduction. Bacteriophage P22HTint was routinely used for transduction using standard methods (Davis et al., "A Man. for Genet. Eng.-Adv. Bacterial Genetics". Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., (1979)). An overnight culture of the donor strain was diluted 1:20 into prewarmed Luria broth, grown for 60 minutes with shaking at 37° C. and then infected with P22HTint at a multiplicity of 0.01. The infection mixture was shaken overnight for approximately 15 h, chloroform added and allowed to shake an additional 10 min at 37° C., and the suspension centrifuged (Sorvall RC5C, SS-34 rotor, 7,000 rpm, 10 min) to remove bacterial debris. The supernatant fluid containing the phage (ca. 10¹⁰ /ml) was stored at 4° C. over chloroform. Tetracycline to a concentration of 12.5 μg/ml was used to select for transduction of Tn10 insertions and Tn10-induced mutations.

Fusaric acid selection for loss of Tn10. The media and methods described by Maloy and Nunn (J. Bacteriol. 145:1110-1112, (1981)) were used. Strains with Tn10-induced mutations were grown overnight in L broth containing 12.5 mg tetracycline/ml at 37° C. to approximately 5×10⁸ CFU/ml. Cultures were then diluted 1:40 into prewarmed L broth without tetracycline and aerated at 37° C. to a titer of about 2×10⁹ CFU/ml. Suitable numbers of cells (i.e. 10⁷ -10⁸) diluted in BSG were plated on fusaric acid-containing medium and incubated 48 h at 37° C. Fusaric acid-resistant isolates were purified on the same selective medium. Single isolates were picked, grown and tested for tetracycline sensitivity on Penassay agar with and without 12.5 μg tetracycline/ml.

Mice. Female BALB/c mice (6 to 8 weeks old) (Sasco, Omaha, Nebr.) were used for infectivity and/or immunization experiments. Animals were held for one week in a quarantined room prior to being used in experiments. Experimental mice were placed in Nalgene filter-covered cages with wire floors. Food and water were given ad libitum. The animal room was maintained at 22°-23° C. with a period of 12 h illumination.

Animal infectivity. The virulence of S. typhimurium strains was determined following peroral (p.o.) or intraperitoneal (i.p.) inoculation. Bacteria for inoculation in mice were grown overnight as standing cultures at 37° C. in L broth. These cultures were diluted 1:50 into prewarmed L broth and aerated at 37° C. for approximately 4 h to an OD₆₀₀ of about 0.8-1.0. The cells were concentrated 50-fold by centrifugation in a GSA rotor at 7,000 rpm for 10 min at 4° C. in a Sorvall RC5C centrifuge followed by suspension in BSG. Suitable dilutions were plated on Penassay agar for titer determination and on MacConkey agar with 1% maltose to verify the Cya/Crp phenotype. For all p.o. inoculations with S. typhimurium, mice were deprived of food and water for 4 h prior to infection. They were then given 30 ml of 10% (w/v) sodium bicarbonate using a Pipetman P200 10-15 min prior to p.o. feeding of 20 μl of S. typhimurium suspended in BSG using a Pipetman P20. Food and water were returned 30 min after oral inoculation. Morbidity and mortality of mice were observed over a 30-day period. Intraperitoneal inoculation of unfasted BALB/c mice was performed using a 26-gauge 3/8" needle to deliver 100 μl of S. typhimurium bacterial suspension diluted in BSG. Morbidity and mortality of mice were observed over a 30-day period.

Evaluation of protective immunity. In initial experiments, any mice that survived infection with any S. typhimurium mutant strain for 30 days were challenged on day 31 with 10³ -10⁴ times the LD₅₀ dose of the wild-type mouse-virulent S. typhimurium parent strain by the p.o. route. Subsequently, groups of mice were perorally immunized with various doses of a virulent mutants and then challenged with various doses of virulent wild-type parent cells at various times after the initial immunization. Morbidity and mortality were observed throughout the experiment and for a least 30 days after challenge with the wild-type parent.

Isolation of S. typhimurium strains with Δcya-12 and Δcrp-11 mutations. The wild-type, mouse-passaged virulent S. typhimurium SL1344 strain .sub.χ 3339 were genetically modified as described below, using classical genetic methods similar to those described in Curtiss and Kelly (1987) Infect. Immun., 55:3035-3043. The strategy consisted of transducing the original crp-773::Tn10 mutation from PP1037 and the original cya::Tn10 mutation from PP1002 into the highly virulent and invasive S. typhimurium SL1344 strain .sub.χ 3339 and screening numerous independent fusaric acid resistant, tetracycline sensitive deletion mutants for complete avirulence and highest immunogenicity in mice, as well as for greatest genotypic stability.

Transduction of the Tn10 insertions in the crp and cya genes was facilitated by first making a high-titer bacteriophage P22HTint lysate on the S. typhimurium strain PP1037 containing the crp-773::Tn10 mutation and another lysate on the S. typhimurium strain PP1002 containing the cya::Tn10 mutation. The resulting P22HTint lysates were subsequently used to infect the recipient S. typhimurium .sub.χ 3339 at a multiplicity of 0.3 to transduce it to tetracycline resistance with screening for a maltose-negative phenotype. The phage-bacteria infection mixtures were incubated for 20 min at 37° C. before 100 μl samples were spread onto MacConkey agar (Difco Laboratories, Detroit, Mich.) containing 1% maltose (final concentration) supplemented with 12.5 μg tetracycline/ml. After approximately 26 h incubation at 37° C. a tetracycline-resistant, maltose-negative colony resulting from the P22HTint (PP1037)→.sub.χ 3339 infection and a tetracycline-resistant, maltose-negative colony resulting from the P22HTint (PP1002)→.sub.χ 3339 infection were picked into 0.5 ml BSG and streaked onto the same selective media. The resulting .sub.χ 3339 derivatives were designated .sub.χ 3604 (cya::Tn10) and .sub.χ 3605 (crp-773::Tn10) (Table 1.A.).

                                      TABLE 1                                      __________________________________________________________________________     Bacterial strains                                                              Strain                                                                         number                                                                               Relevant genotype Derivation                                             __________________________________________________________________________     A. E. coli                                                                     CA8445                                                                                ##STR1##         Schroeder and Dobrogosz, J. Bacteriol.                                         167:616-622 (1986).                                    χ6060                                                                             ##STR2##         Goldschmidt, Thoren-Gorden and Curtiss, J.                                     Bacteriol. 172:3988-4001 (1990).                       B. S. typhimurium                                                              798   wild-type prototroph                                                                             Received from R. Wood, NADC, Ames, IA, as                                      a swine isolate.                                       #30875                                                                               wild-type prototroph                                                                             Received from P. McDonough, Cornell Univ.                                      NY as a horse isolate.                                 DU8802                                                                                ##STR3##         Sanderson and Roth, Microbiol. Rev.                                            42:485-532 (1988).                                     PP1002                                                                                ##STR4##         Postma, Keizer and Koolwijk, J Bacteriol.                                      168:1107-1111 (1986).                                  PP1037                                                                                ##STR5##         Postma, Keizer and Koolwijk, supra.                    SGSC452                                                                               ##STR6##         Sanderson and Roth, 1988 supra.                        TT172                                                                                 ##STR7##         Sanderson and Roth, 1986 supra.                        TT2104                                                                                ##STR8##         Sanderson and Roth, supra.                             χ3000                                                                            LT2-Z prototroph  Gulig and Curtiss, Infect. Imun, 55:2891-                                      2901 (1987).                                           χ3140                                                                            SR-11 wild-type prototroph                                                                       Gulig and Curtiss, 1987 supra.                         χ3306                                                                             ##STR9##         Gulig and Curtiss, 1987 supra.                         χ3385                                                                             ##STR10##        Tinge and Curtiss, J. Bacteriol. 172: in press                                 (1990).                                                χ3339                                                                             ##STR11##        Smith et al., Am. J. Vet. Res. 43:59-66                                        (1984).                                                χ3520                                                                             ##STR12##        ATCC53681; Asd.sup.-  tetracycline-resistant                                   derivative of χ3000.                               χ3604                                                                             ##STR13##        P22HT --int(PP1002) → χ3339 with                                    selection                                                                      for tetracycline resistance (Mal.sup.-).               χ3605                                                                             ##STR14##        P22HT --int(PP1037) → χ3339 with                                    selection                                                                      for tetracycline resistance (Mal.sup.-).               χ3615                                                                             ##STR15##        Fusaric acid-resistant, tetracycline-                                          sensitive Mal.sup.-  derivative of χ3604.          χ3622                                                                             ##STR16##        Fusaric acid-resistant, tetracycline-                                          sensitive Mal.sup.-  Cys.sup.-  Arg.sup.-                                      derivative of                                                                  χ3605.                                             χ3623                                                                             ##STR17##        Fusaric acid-resistant, tetracycline-                                          sensitive Mal.sup.-  derivative of χ3605.          χ3670                                                                             ##STR18##        χ3385 transformed with pSD110 from CA8445                                  with selection for ampicillin resistance,                                      Mal.sup.+.                                             χ3706                                                                             ##STR19##        χ3622 transformed with pSD110 from CA8445                                  with selection for ampicillin resistance,                                      Mal.sup.+.                                             χ3711                                                                             ##STR20##        P22HT  --int(χ3738) → χ3615 with                                selection for                                                                  tetracycline resistance, Mal.sup.-.                    χ3712                                                                             ##STR21##        P22HT --int(χ3741) → χ3622 with                                 selection for                                                                  tetracycline resistance, Mal.sup.-, (Cys.sup.-,                                Arg.sup.-).                                            χ3722                                                                             ##STR22##        P22HT --int(χ3711) → χ3706 with                                 selection for tetracycline resistance                                          (Mal.sup.-).                                           χ3723                                                                             ##STR23##        Fusaric acid-resistant, tetracycline- sensitive,                               ampicillin-resistant, Mal.sup.-,                                               Cys.sup.-, Arg.sup.-  derivative of χ3723.         χ3724                                                                             ##STR24##        Ampicillin-sensitive derivative of χ3723;                                  pSD110 cured by serial passage in L broth                                      at 37° C.                                       χ3730                                                                             ##STR25##        Asd.sup.-  Tc.sup.s derivative if SGSC452.             χ3731                                                                             ##STR26##        Spleen isolate of χ3706 from BALB/c mouse.         χ3738                                                                             ##STR27##        P22HT --int(TT2104) → χ3000 with                                    selection                                                                      for tetracycline resistance.                           χ3741                                                                             ##STR28##        P22HT --int(DU8802) → χ3000 with                                    selection                                                                      for tetracycline resistance.                           χ3761                                                                            UK-1 wild-type prototroph                                                                        ATCC68169; Spleen isolate of #30875 from                                       White leghorn chick.                                   χ3773                                                                             ##STR29##        P22HT --int(χ3741) → χ3623 with                                 selection for                                                                  tetracycline resistance (Mal.sup.-).                   χ3774                                                                             ##STR30##        χ3623 transformed with pSD110 from CA8445                                  with selection for ampicillin resistance,                                      Mal.sup.+.                                             χ3777                                                                             ##STR31##        P22HT --int(χ3712) → 798 with                                       selection for                                                                  tetracycline resistance, Mal.sup.-, (Cys.sup.-,                                Arg.sup.-).                                            χ3779                                                                             ##STR32##        P22HT --int(χ3712) → #30875 with                                    selection                                                                      for tetracycline resistance, Mal.sup.-,                                        (Cys.sup.-, Arg.sup.-).                                χ3784                                                                             ##STR33##        Fusaric acid-resistant, tetracycline-                                          sensitive, Mal.sup.-, Cys.sup.-, Arg.sup.-                                     derivative of                                                                  χ3779.                                             χ3806                                                                             ##STR34##        Fusaric acid-resistant, tetracycline-                                          sensitive, ampicillin-resistant, Mal.sup.-,                                    Cys.sup.-, Arg.sup.-  derivative of χ3777.         χ3825                                                                             ##STR35##        P22HT --int(χ3773) → 798 with                                       selection for                                                                  tetracycline resistance, Mal.sup.-.                    χ3828                                                                             ##STR36##        P22HT --int(χ3773) → UK-1 with                                      selection for                                                                  tetracycline resistance, Mal.sup.-.                    χ3876                                                                             ##STR37##        Fusaric acid-resistant, tetracycline-                                          sensitive, Mal.sup.-  derivative of χ3825.         χ3901                                                                             ##STR38##        P22HT --int(χ3670) → χ3806 with                                 selection for ampicillin resistance, Mal.sup.+,                                (Cys.sup.-, Arg.sup.-).                                χ3902                                                                             ##STR39##        P22HT --int(χ3711) → χ3901 with                                 selection for tetracycline resistance, Mal.sup.-,                              (Cys.sup.-, Arg.sup.-).                                χ3910                                                                             ##STR40##        P22HT --int(TT172) → χ3339 with                                     selection for                                                                  tetracycline resistance, Cys.sup.-.                    χ3931                                                                             ##STR41##        Fusaric acid-resistant, tetracycline-                                          sensitive, Mal.sup.-, Cys.sup.-, (Arg.sup.+)                                   derivative                                                                     of χ3910.                                          χ 3936                                                                            ##STR42##        P22HT --int(χ3711) → χ3774 with                                 selection for tetracycline resistance,                                         Mal.sup.-.                                             χ3937                                                                             ##STR43##        Fusaric acid-resistant, tetracycline sensitive,                                Mal.sup.-  derivative of χ3936.                    χ3938                                                                             ##STR44##        P22HT --int(χ3670) → χ3876 with                                 selection for                                                                  ampicillin resistance, Mal.sup.+.                      χ3939                                                                             ##STR45##        Ampicillin-sensitive derivative of χ3937;                                  pSD110 cured by serial passage in L broth                                      at 37° C.                                       χ3945                                                                             ##STR46##        P22HT --int(χ3670) → χ3784 with                                 selection for ampicillin resistance, Mal.sup.+.        χ3954                                                                             ##STR47##        Fusaric acid-resistant, tetracycline-                                          sensitive, Mal.sup.-  derivative of χ3828.         χ3955                                                                             ##STR48##        P22HT --int(χ3670) → χ3931 with                                 selection for                                                                  ampicillin resistance, Mal.sup.+, (Cys.sup.-,                                  Arg.sup.+).                                            χ3956                                                                             ##STR49##        P22HT --int(χ3711) → χ3945 with                                 selection for tetracycline resistance, Mal.sup.-,                              Cys.sup.-, Arg.sup.-.                                  χ3957                                                                             ##STR50##        Fusaric acid-resistant, tetracycline- sensitive,                               Mal.sup.-, Cys.sup.-, Arg.sup.-  derivative of                                 χ3956.                                             χ3958                                                                             ##STR51##        Ampicillin-sensitive derivative of χ3957;                                  pSD110 cured by serial passage in L broth                                      at 37° C.                                       χ3961                                                                             ##STR52##        P22HT --int(χ3670) → χ3954 with                                 selection for                                                                  ampicillin resistance, Mal.sup.+.                      χ3962                                                                             ##STR53##        P22HT --int(χ3711) → χ3961 with                                 selection for tetracycline resistance,                                         Mal.sup.-.                                             χ3978                                                                             ##STR54##        P22HT --int(χ3711) → χ3938 with                                 selection for tetracycline resistance,                                         Mal.sup.-.                                             χ3985                                                                             ##STR55##        ATCC68166; Fusaric acid-resistant, tetracycline-se                             nsitive, Mal.sup.-  derivative of                                              χ3962 cured of pSD110.                             χ4038                                                                             ##STR56##        Fusaric acid-resistant tetracycline- sensitive                                 Mal.sup.-, Cys.sup.-, Arg.sup.-  derivative of                                 χ3902 cured of pSD110. -                           χ4039                                                                             ##STR57##        Fusaric acid-resistant, tetracycline- sensitive                                Mal.sup.-  derivative of χ3978 cured                                       of pSD110.                                             χ4063                                                                             ##STR58##        P22HT --int(Tn --10 library) → χ3306                                with                                                                           selection for tetracycline resistance,                                         Arg.sup.-.                                             χ4071                                                                             ##STR59##        P22HT --int(Tn --10 library) → χ3306                                with                                                                           selection for tetracycline resistance,                                         Arg.sup.-.                                             χ4246                                                                             ##STR60##        P22HT --int(χ3712) → 798 with                                       selection for                                                                  tetracycline resistance, Mal.sup.-, (Cys.sup.-                                 Arg.sup.-).                                            χ4247                                                                             ##STR61##        P22HT --int(χ3670) →  χ4246 with                                selection for ampicillin resistance, Mal.sup.+,                                (Cys.sup.-  Arg.sup.-).                                χ4248                                                                             ##STR62##        P22HT --int(χ3712) → ATCC68169 (UK-1)                               with                                                                           selection for tetracycline resistance,                                         Mal.sup.-, (Cys.sup.-  Arg.sup.-).                     χ4262                                                                             ##STR63##        P22HT --int(χ3670) → χ4248 with                                 selection for ampicillin resistance, Mal.sup.+,                                (Cys.sup.-  Arg.sup.-).                                C. S. typhi                                                                    Ty2   Type E1 Cys.sup.- Louis Baron, Walter Reed Army Institute of                                     Research.                                              ISP1820                                                                              Type 46 Cys.sup.- Center for Vaccine Development, Baltimore,                                     MD; 1983 isolate from Chilean patient.                 ISP2822                                                                              Type E1 Cys.sup.- Center for Vaccine Development, Baltimore,                                     MD; 1983 isolate from Chilean patient.                 χ3791                                                                             ##STR64##        P22HT --int(χ3712) → ISP2822 with                                   selection                                                                      for tetracycline resistance (Mal.sup.-,                                        Cys.sup.-,                                                                     Arg.sup.-, Vi.sup.+).                                  χ3792                                                                             ##STR65##        P22HT --int(χ3712) → Ty2 with with                                  selection for                                                                  tetracycline resistance (Mal.sup.-, Cys.sup.-,                                 Arg.sup.-                                                                      Vi.sup.+).                                             χ3802                                                                             ##STR66##        Fusaric acid-resistant, tetracycline-                                          sensitive Mal.sup.-  derivative of χ3791                                   (Vi.sup.+).                                            χ3803                                                                             ##STR67##        Fusaric acid-resistant, tetracycline-                                          sensitive Mal.sup.-  derivative of χ3791                                   (Vi.sup.+).                                            χ3824                                                                             ##STR68##        χ3803 electro-transformed with pSD110 from                                 χ3670 with selection for ampicillin                                        resistance (Mal.sup.+, Cys.sup.-, Arg.sup.-,                                   Vi.sup.+).                                             χ3845                                                                             ##STR69##        χ3802 electro-transformed with pSD110 from                                 χ3670 with selection for ampicillin                                        resistance (Mal.sup.+, Cys.sup.-, Arg.sup.-,                                   Vi.sup.+).                                             χ3852                                                                             ##STR70##        P22HT --int(Δ3773) → ISP2822 with                                 selection                                                                      for tetracycline resistance (Mal.sup.-, Vi+).          χ3853                                                                             ##STR71##        P22HT --int(χ3773) → Ty2 with                                       selection for                                                                  tetracycline resistance (Mal.sup.-, Vi.sup.+).         χ3877                                                                             ##STR72##        Fusaric acid-resistant, tetracycline-                                          sensitive Mal.sup.-  derivative of χ3852                                   (Vi.sup.+).                                            χ3878                                                                             ##STR73##        Fusaric acid-resistant, tetracycline-                                          sensitive Mal.sup.-  derivative of χ3853                                   (Vi.sup.+).                                            χ3879                                                                             ##STR74##        P22HT --int(χ3670) → Δ3877 with                               selection for                                                                  ampicillin resistance (Mal.sup.+, Vi.sup.+).           χ3880                                                                             ##STR75##        P22HT --int(χ3670) → χ3878 with                                 selection for                                                                  ampicillin resistance (Mal.sup.+, Vi.sup.+).           χ3919                                                                             ##STR76##        P22HT --int(χ3711) → χ3824 with                                 selection for tetracycline resistance (Mal.sup.-,                              Vi.sup.+).                                             χ3920                                                                             ##STR77##        P22HT --int(χ3711) → χ3845 with                                 selection for tetracycline resistance (Mal.sup.-,                              Vi.sup.+).                                             χ3921                                                                             ##STR78##        P22HT --int(χ3711) → χ3879 with                                 selection for tetracycline resistance (Mal.sup.-,                              Vi.sup.+).                                             χ3922                                                                             ##STR79##        P22HT --int(χ3711) → χ3880 with                                 selection for tetracycline resistance (Mal.sup.-,                              Vi.sup.+).                                             χ3924                                                                             ##STR80##        Fusaric acid-resistant, tetracycline- sensitive                                Mal.sup.-  derivative of χ3919 cured                                       of pSD110 (Vi.sup.+).                                  χ3925                                                                             ##STR81##        Fusaric acid-resistant, tetracycline- sensitive                                Mal.sup.-  derivative of χ3920 cured                                       of pSD110 (Vi.sup.+).                                  χ3926                                                                             ##STR82##        Fusaric acid-resistant, tetracycline- sensitive                                Mal.sup.-  derivative of χ3921 cured                                       of pSD110 (Vi.sup.+).                                  χ3927                                                                             ##STR83##        Fusaric acid-resistant, tetracycline- sensitive                                Mal.sup.-  derivative of χ3922 cured                                       of pSD110 (Vi.sup.+).                                  χ3940                                                                             ##STR84##        Flagella-positive, motile derivative of χ3925                              (Vi.sup.+).                                            χ4073                                                                             ##STR85##        Flagella-positive, motile derivative of χ3924                              (Vi.sup.+).                                            χ4296                                                                             ##STR86##        P22HT --int(χ3520) → χ3927 with                                 selection for tetracycline resistance and                                      screening for Asd.sup.-, Mal.sup.-, Vi.sup.+.          χ4297                                                                             ##STR87##        Fusaric acid-resistant, tetracycline- sensitive                                Asd.sup.-, Mal.sup.-  derivative of χ4296                                  (Vi.sup.+).                                            χ4298                                                                             ##STR88##        P22HT --int(χ3773) → ISP1820 with                                   selection                                                                      for tetracycline resistance (Mal.sup.-,                                        Vi.sup.+).                                             χ4299                                                                             ##STR89##        Fusaric acid-resistant, tetracycline-                                          sensitive Mal.sup.-  derivative of χ4298                                   (Vi.sup.+).                                            χ4300                                                                             ##STR90##        P22HT --int(χ3670) → Δ4299 with                               selection for                                                                  ampicillin resistance (Mal.sup.+, Vi.sup.+ ).          χ4316                                                                             ##STR91##        P22HT --int(χ3670) → χ4300 with                                 selection for tetracycline resistance                                          (Mal.sup.-, Vi.sup.+).                                 χ4322                                                                             ##STR92##        Fusaric acid-resistant, tetracycline- sensitive                                Mal.sup.-  derivative of χ4316 cured                                       of pSD110 (Vi.sup.+).                                  χ4323                                                                             ##STR93##        Flagella-positive, motile derivative of χ4322                              (Vi.sup.+)                                             χ4324                                                                             ##STR94##        P22HT --int(χ3712) → ISP1820 with                                   selection                                                                      for tetracycline resistance (Mal.sup.-,                                        Cys.sup.-,                                                                     Arg.sup.-, Vi.sup.+).                                  χ4325                                                                             ##STR95##        Fusaric acid-resistant, tetracycline-                                          sensitive Mal.sup.-  derivatice of χ4324                                   (Vi.sup.+).                                            χ4331                                                                             ##STR96##        P22HT --int(χ3670) → χ4325 with                                 selection for ampicillin resistance (Mal.sup.+,                                Vi.sup.+).                                             χ4340                                                                             ##STR97##        P22HT --int(χ3711) → χ4331 with                                 selection for tetracycline resistance (Mal.sup.-,                              Vi.sup.+).                                             χ4345                                                                             ##STR98##        Fusaric acid-resistant, tetracycline-  sensitive                               Mal.sup.-  derivative of χ4340 cured                                       of pSD110 (Vi.sup.+).                                  χ4346                                                                             ##STR99##        Flagella-positive, motile derivative of χ4345                              (Vi.sup.+).                                            __________________________________________________________________________

Strains .sub.χ 3604 and .sub.χ 3605 were grown in L broth+12.5 μg tetracycline/ml and 100 μl samples of each strain diluted 1:10 into buffered saline with gelatin (BSG) were spread onto 10 plates of fusaric acid-containing (FA) media (Maloy and Nunn, 1981). The plates were incubated approximately 36 h at 37° C. Five fusaric acid-resistant colonies from each plate were picked into 0.5 ml BSG and purified on FA media. Purified fusaric acid-resistant colonies were picked into L broth and grown at 37° C. to turbidity and checked for loss of Tn10 (tetracycline sensitivity). One tetracycline-sensitive derivative was selected from each of the ten platings on FA media and characterized for complete LPS (by P22HTint sensitivity), auxotrophy or prototrophy, stability of the gene deletion, and reversion to tetracycline resistance. This procedure resulted in ten independently isolated Δcya mutants from .sub.χ 3604 and ten independently isolated Δcrp mutants from .sub.χ 3605.

Genetic stability of avirulent mutants. Strains to be orally administered as live vaccines must have complete stability with regard to both their avirulence and their immunogenic attributes. When 50-fold concentrated cultures and various dilutions (˜10⁹, 10⁷, 10⁵, 10³ CFU/plate) of each of the ten independent Δcya mutants and each of the ten independent Δcrp mutants were plated on minimal agar media (supplemented with 22 μg cysteine/ml and 22 μg arginine/ml) containing 0.5% maltose, melibiose, xylose, glycerol, or rhamnose that should not support their growth, revertants and mutants were not detected. One set of duplicate plates were UV-irradiated (5 joules/meter² /sec) and incubated at 37° C. in the dark. The other set of plates was incubated at 37° C. with illumination. Revertants and mutants were not detected after a 48 h growth period. An investigation was also conducted as to whether tetracycline-resistant revertants/mutants could be recovered from the fusaric acid resistant Δcya and Δcrp mutants at frequencies higher than could be observed for the tetracycline-sensitive wild-type parental strain. In all cases, such tetracycline-resistant revertants/mutants were not observed.

Virulence and immunogenicity of Δcrp and Δcya mutants. The resulting ten Δcrp and ten Δcya mutants were screened in BALB/c mice by peroral inoculation to determine the lowest virulence and disease symptomology as revealed by the appearance of the coat (scruffy versus smooth), appetite, and activity (high or low). Five mice per group were p.o. inoculated with ˜10⁹ CFU of each of the independent cya or crp deletion mutants. Animals were scored based on the above criteria and on day 30 of the experiment the survivors were challenged with 10⁸ CFU of the wild-type virulent parent strain .sub.χ 3339. In three of the twenty groups infected with the cya or crp deletion mutants, five of five mice survived the initial infection with the Δcya-12, Δcrp-11 and Δcrp-10 mutants and were also completely protected against 10⁴ LD₅₀ s of the wild-type challenge. One group in particular, the Δcrp-10 mutant, was unequalled in avirulence, immunogenicity and stability. After repeating these experiments, mice never appeared affected by any dose given p.o. or i.p. of the Δcrp-10 mutant (see Example 3, Table 6).

Properties of selected mutant strains. .sub.χ 3615, .sub.χ 3622 and .sub.χ 3623 with the Δcya-12, Δcrp-10 and Δcrp-11 mutations, respectively, were judged to be least virulent, highly immunogenic and extremely stable phenotypically and genotypically. Data on the phenotypic properties of these strains is given in Table 2. Table 3 presents data on the avirulence and immunogenicity of these strains in comparison to results with the virulent wild-type parent .sub.χ 3339 and strains .sub.χ 3604 and .sub.χ 3605 with the cya::Tn10 and crp-773::Tn10 mutations, respectively. In addition to requiring histidine, which is due to the hisG mutation in the parental .sub.χ 3339, the Δcrp-10 mutation imposed on .sub.χ 3622 requirements for the amino acids arginine and cysteine. The bases for this observation and further analysis of the properties of the Δcrp-10 mutation are given in Example 3.

                                      TABLE 2                                      __________________________________________________________________________     Phenotype characteristics of S. typhimurium Δcya and Δcrp          strains                                                                        Strain and  Carbohydrate fermentation and use.sup.b                                                                Auxotrophy                                 genotype P22.sup.a                                                                         Mal                                                                               Mtl                                                                               Ino                                                                               Srl                                                                              Rha                                                                               Mel                                                                               Gal Glc                                                                               His                                                                               Arg                                                                               Cys                                  __________________________________________________________________________     χ3339                                                                          wild type                                                                           S  +  +  +  + +  +  +   +  -  +  +                                    χ3615                                                                           ##STR100##                                                                         S  -  -  -  - -  -  +/- +  -  +  +                                    χ3622                                                                           ##STR101##                                                                         S  -  -  -  - -  -  +/- +  -  -  -                                    χ3623                                                                           ##STR102##                                                                         S  -  -  -  - -  -  + /-                                                                               +  -  +  +                                    __________________________________________________________________________      ##STR103##                                                                     .sup.b Fermentation on MacConkey Base agar media and API 20E and growth o      MA + 0.5% of carbon source.                                              

                                      TABLE 3                                      __________________________________________________________________________     Virulence and immunogenicity of S. typhimurium cya::Tn10, crp::Tn10            Δcya-12, Δcrp-10 and Δcrp-11 mutants in BALB/c mice                       P.O. immunization                                                                           WIld-type P.O. challenge                             Strain                                                                               Relevant      Survival     Survival                                      number                                                                               genotype                                                                              Dose (CFU)                                                                            live/total                                                                           Dose (CFU)                                                                            live/total                                    __________________________________________________________________________     χ3339                                                                            wild type                                                                             --     --    6.0 × 10.sup.4                                                                  2/5                                           χ3604                                                                             ##STR104##                                                                           6.2 × 10.sup.8                                                                  5/5   8.8 × 10.sup.8                                                                  4/5                                           χ3605                                                                             ##STR105##                                                                           6.8 × 10.sup.8                                                                  5/5   8.8 × 10.sup.8                                                                  5/5                                           χ3615                                                                             ##STR106##                                                                           2.2 × 10.sup.9                                                                  5/5   3.2 × 10.sup.8                                                                  5/5                                           χ3622                                                                             ##STR107##                                                                           1.5 × 10.sup.9                                                                  5/5   3.2 × 10.sup.8                                                                  5/5                                           χ3623                                                                             ##STR108##                                                                           4.6 × 10.sup.8                                                                  5/5   8.8 × 10.sup.8                                                                  5/5                                           __________________________________________________________________________

Example 2

This example describes the construction of avirulent microbes by the introduction of deletion mutations affecting cAMP synthesis and utilization and the characterization of strains with two deletion mutations for stability of phenotype, complete avirulence and high immunogenicity.

Bacterial strains. The Escherichia coli and Salmonella typhimurium strains used are listed in Table 1.A. and B. The maintenance and storage of these strains are as described in Example 1.

Media. Complex media for routine cultivation, enumeration and identification of bacteria are as described in Example 1.

Transduction and fusaric acid selection for loss of Tn10. The media and methods are as described in Example 1.

Animal infectivity and evaluation of protective immunity. The virulence and immunogenicity of S. typhimurium strains were determined as described in Example 1.

Construction of S. typhimurium strains with Δcya-12 and Δcrp-11 deletion mutations. The best vaccine strains in terms of efficacy are likely to result from the attenuation of highly virulent strains that display significant colonizing ability and invasiveness. The criteria for selection of these highly pathogenic S. typhimurium wild-type strains such as SL1344 (.sub.χ 3339), UK-1 (.sub.χ 3761) and 798 included low LD₅₀ values (see Table 4) in mouse virulence assays, antibiotic sensitivity, possession of the virulence plasmid, ease of genetic manipulation (bacteriophage P22HTint or P1 sensitivity, transformability and ease of receiving mobilized plasmids), and colicin sensitivity.

The wild-type, virulent S. typhimurium strains SL1344 (.sub.χ 3339), 798 and UK-1 (.sub.χ 3761) were genetically modified as described below, using classical genetic methods similar to those described in Curtiss and Kelly (1987). The strategy consists of mobilizing deletions of crp and cya genes that have been isolated and characterized in S. typhimurium SL1344 (as described in Example 1) by placing the transposon Tn10 (encoding tetracycline resistance) nearby the Δcya-12 or Δcrp-11 mutation and transducing the linked traits into the highly virulent S. typhimurium strains UK-1 .sub.χ 3761, 798 and SL1344 .sub.χ 3339 via P22HTint-mediated transduction with selection for tetracycline resistance and screening for a maltose-negative phenotype. The zhc-1431::Tn10 linked to Δcrp-11 and zid-62::Tn10 linked to Δcya-12 were used for this purpose. Neither insertion alone affects the virulence of S. typhimurium.

Transduction of the gene deletions with the linked transposon was facilitated by first making a high-titer bacteriophage P22HTint lysate on the S. typhimurium strain .sub.χ 3773 containing the Δcrp-11 and zhc-1431::Tn10 mutations and another lysate on the S. typhimurium strain .sub.χ 3711 containing the Δcya-12 and zid-62::Tn10 mutations. The resulting P22HTint lysates were then used to transduce the genetic traits into the wild-type recipient strains .sub.χ 3339, 798 and .sub.χ 3761.

P22HTint propagated on S. typhimurium .sub.χ 3773 (Δcrp-11 zhc-1431::Tn10) was used to transduce the virulent strains to tetracycline resistance with screening for Mal⁻. The phage-bacteria infection mixtures were incubated for 20 min at 37° C. before 100 μl samples were spread onto MacConkey agar (Difco Laboratories, Detroit, Mich.) containing 1% maltose (final concentration) supplemented with 12.5 μg tetracycline/ml. After approximately 26 h incubation at 37° C., tetracycline resistant Mal⁻ transductants were picked and purified onto the same medium. The resulting 798 derivative was designated .sub.χ 3825 and the UK-1 derivative was designated .sub.χ 3828. Strains .sub.χ 3773, .sub.χ 3825 and .sub.χ 3828 have the genotype Δcrp-11 zhc-1431::Tn10 (Table 1.B.). These strains were grown in L broth+12.5 μg tetracycline/ml and each were diluted 1:10 into buffered saline with gelatin (BSG), 100 μl of each were spread onto fusaric acid-containing (FA) media (Maloy and Nunn, 1981) and the plates were incubated approximately 36 h at 37° C. Fusaric acid-resistant colonies of each strain were picked into 0.5 ml BSG and purified onto FA media. Purified fusaric acid-resistant colonies were picked into L broth and grown at 37 ° C. to turbidity and checked for loss of Tn10 (tetracycline sensitivity), presence of complete LPS and auxotrophy. The new strains were designated .sub.χ 3876 (798) and .sub.χ 3954 (UK-1) which both have the genotype Δcrp-11 Δ[zhc-1431::Tn10] and .sub.χ 3623 (SL1344 Δcrp-11 was originally isolated as described in Example 1) (Table 1.B.).

Since the phenotype of Cya⁻ and Crp⁻ mutants are the same (Mal⁻, Stl⁻, Mtl⁻, etc.), the plasmid, pSD110, carrying the cloned crp⁺ gene and conferring ampicillin resistance (Schroeder and Dobrogosz, J. Bacteriol 167:616-622 (1986)), was used to temporarily complement the Δcrp mutation in the chromosome enabling the identification of the Δcya mutation when introduced via transduction. L broth grown cultures of .sub.χ 3623, .sub.χ 3876 and .sub.χ 3954 were transduced with P22HTint propagated on S. typhimurium .sub.χ 3670, which contains the plasmid pSD110 (Table 1.B.). Selection was made on MacConkey agar+1% maltose+100 μg ampicillin/ml. After 26 h, an ampicillin-resistant, Mal⁺ colony of each strain was picked and purified on MacConkey agar+1% maltose agar+100 μg ampicillin/ml and designated .sub.χ 3938 (798) and .sub.χ 3961 (UK-1) which both have the genotype Δcrp-11 Δ[zhc-1431::Tn10] pSD110⁺ and .sub.χ 3774 (SL1344) which has the genotype Δcrp-11 pSD110⁺.

Strains .sub.χ 3774, .sub.χ 3938 and .sub.χ 3961 were grown in L broth+100 μg ampicillin/ml and were each independently transduced with P22HTint propagated on .sub.χ 3711 to introduce the linked Δcya-12 and zid-62::Tn10 mutations. The transduction mixtures were plated on MacConkey agar+1% maltose+100 μg ampicillin/ml+12.5 μg tetracycline/ml. Ampicillin-resistant (pSD110⁺), tetracycline-resistant (zid-62::Tn10), Mal⁻ (Δcya) colonies were picked and purified on MacConkey agar+1% maltose+100 μg ampicillin/ml+12.5 μg tetracycline/ml. Purified colonies were picked into L broth, grown to turbidity and the strains checked for complete LPS and auxotrophy. The resulting strains were designated .sub.χ 3978 (798) and .sub.χ 3962 (UK-1) which both have the genotype Δcrp-11 Δ[zhc-1431::Tn10] pSD110⁺ Δcya-12 zid-62::Tn10 and .sub.χ 3936 (SL1344) which has the genotype Δcrp-11 pSD110⁺ Δcya-12 zid-62::Tn10. Cultures of .sub.χ 3936, .sub.χ 3978 and .sub.χ 3962 were grown in L broth+100 μg ampicillin/ml+12.5 μg tetracycline/ml to turbidity, diluted 1:10 into BSG, and 100 μl samples of each culture spread onto fusaric acid-containing media and incubated approximately 36 h at 37° C. Fusaric acid-resistant colonies of each strain were picked and purified onto FA medium. Purified FA-resistant colonies were picked into L broth, grown to turbidity and then checked for loss of Tn10 (tetracycline sensitivity), complete LPS and auxotrophy. The pSD110 plasmid was usually lost spontaneously from the strains during this process to result in ampicillin sensitivity, except for the SL1344 derivative which involved two steps to eliminate pSD110. The final strains were designated .sub.χ 4039 (798) and .sub.χ 3985 (UK-1) which both have the genotype Δcrp-11 Δ[zhc-1431::Tn10] Δcya-12 Δ[zid-62::Tn10] and .sub.χ 3939 (SL1344) which has the genotype Δcrp-11 Δcya-12 Δ[zid-62::Tn10] (Table 1.B.).

Genotypic and phenotypic stability of avirulent mutants. Methods for determining stability of genetic traits are as described in Example 1. All genotypic and phenotypic traits due to the Δcya Δcrp mutations were completely stable except motility. Although synthesis of functional flagella and display of motility is dependent on wild-type cya and crp gene functions, a suppressor mutation in the cfs (constitutive flagellar synthesis) gene can easily be selected to cause flagella synthesis and motility to be independent of cya and crp gene functions. In S. typhimurium Δcya Δcrp strains, motile variants were readily selected during the strain construction process. Since immunity to flagellar antigens may be protective, motile variants of all vaccine strains were selected.

S. typhimurium group B O-antigen synthesis was confirmed by slide agglutination with antisera (Difco Laboratories, Detroit, Mich.) and by P22HTint bacteriophage sensitivity by the Luria soft agar overlay technique.

Fermentation of sugars and growth on various carbon sources of the double mutant strains were identical to strains with only Δcya or Δcrp as listed in Table 2. The phenotypes were as expected based on published reports of the requirement for cyclic AMP and the cyclic AMP receptor protein for catabolic activities.

At each step in the construction following selection of a fusaric acid-resistant tetracycline-sensitive derivative, an investigation as to whether tetracycline-resistant revertants/mutants could be recovered at frequencies higher than could be observed for the parental tetracycline-sensitive wild-type strain was conducted. In all cases, such tetracycline-resistant revertants/mutants were not observed.

Virulence of mutant strains for mice. Preliminary information on virulence of S. typhimurium mutant strains was obtained by infecting individual mice with 10⁸ mutant cells perorally and recording morbidity and mortality. Table 4 presents data on morbidity and mortality of mice infected perorally with the S. typhimurium wild-type parent strains, and the Δcya-12 Δcrp-11 derivatives .sub.χ 3985 and .sub.χ 4039.

                                      TABLE 4                                      __________________________________________________________________________     Virulence of S. typhimurium Δcya-12, Δcrp-11, Δcya-12,       and Δcrp-11 Strains                                                      After Inoculation of BALB/c Mice with S. typhimurium Δcya-12 and/or      Δcrp-11 Strains                                                                              Inoculating                                                                          Survival  Approx.                                    Strain                                                                              Relevant Route of                                                                             Dose  live/     wild-type                                                                           Wild-type                             Number                                                                              Genotype Inoculation                                                                          (CFU) Total                                                                               Health.sup.a                                                                        LD.sub.50                                                                           Origin                                __________________________________________________________________________     S. typhimurium                                                                 χ3615                                                                            ##STR109##                                                                             PO    2 × 10.sup.9                                                                   5/5  healthy                                                                             6 × 10.sup.4                                                                  mouse                                 χ3623                                                                            ##STR110##                                                                             PO    5 × 10.sup.8                                                                   5/5  healthy                                                                             6 × 10.sup.4                                                                  mouse                                 χ3985                                                                            ##STR111##                                                                             PO    2 × 10.sup.9                                                                    8/10                                                                               moderate                                                                            1 × 10.sup.5                                                                  horse                                 χ4039                                                                            ##STR112##                                                                             PO    1 × 10.sup.9                                                                   10/10                                                                               healthy                                                                             1 × 10.sup.5                                                                  pig                                   S. typhi                                                                       χ3926                                                                            ##STR113##                                                                             IP.sup.b                                                                             2 × .sup.3                                                                     4/6  healthy                                                                             ˜29                                                                           human                                 χ3927                                                                            ##STR114##                                                                             IP    3 × 10.sup.3                                                                   2/4  healthy                                                                             <20  human                                 __________________________________________________________________________      hu a Healthyno noticeable signs of disease; moderatemoderately ill;            illnoticeably ill.                                                             .sup.b IP-cells delivered in 0.5 ml 5% hog gastric mucin.                

Effectiveness of immunization with avirulent mutants. Table 5 presents data on the ability of the S. typhimurium Δcya Δcrp mutants .sub.χ 3985 and .sub.χ 4039 to induce immunity to subsequent peroral challenge with 10⁴ times the LD₅₀ doses of fully virulent wild-type S. typhimurium cells. Under these high-dose challenges, many of the mice displayed moderate illness with decreased food consumption except mice immunized with .sub.χ 4039 which remained healthy and ate and grew normally.

                                      TABLE 5                                      __________________________________________________________________________     Effectiveness of Immunization with Avirulent S. typhimurium                    Δcya-12 and/or Δcrp-11 Mutants in Protecting Against               Challenge with Wild-type Virulent Parent Strains                                                       Dose (CFU) of                                                         Dose (CFU) of                                                                           Wild-type                                              Strain                                                                              Relevant  Immunizing                                                                              Challenge                                                                               Survival                                      Number                                                                              Genotype  Strain   Strain   live/total                                    __________________________________________________________________________     χ3615                                                                            ##STR115##                                                                              2 × 10.sup.9                                                                      3 × 10.sup.8                                                                      5/5                                           χ3623                                                                            ##STR116##                                                                              5 × 10.sup.8                                                                      3 × 10.sup.8                                                                      5/5                                           χ3985                                                                            ##STR117##                                                                              2 × 10.sup.9                                                                      7 × 10.sup.8                                                                      8/8                                           χ4039                                                                            ##STR118##                                                                              1 × 10.sup.9                                                                      6 × 10.sup.8                                                                      10/10                                         __________________________________________________________________________

Example 3

This Example demonstrates the isolation of an avirulent microbe that possesses a deletion mutation encompassing the crp gene and an adjacent gene which also governs virulence of Salmonella.

Bacterial strains. The Escherichia coli and Salmonella typhimurium strains used are listed in Table 1A and B. The maintenance and storage of these strains are as described in Example 1.

Media. Complex media for routine cultivation, enumeration and identification of bacteria are as described in Example 1.

Transduction and fusaric acid selection for loss of Tn10. The media and methods are as described in Example 1.

Animal infectivity and evaluation of protective immunity. The virulence and immunogenicity of S. typhimurium strains were determined as described in Example 1.

Isolation of S. typhimurium strain with the Δcrp-10 mutation. As described in Example 1, one of ten Δcrp mutations isolated in .sub.χ 3605 conferred auxotrophy for arginine (due to deletion of argD) and cysteine (due to deletion of cysG). The mutation in the S. typhimurium SL1344 strain .sub.χ 3622 was originally referred to as Δcrp-10 but is now designated Δ[crp-cysG]-10 because of the auxotrophy for cysteine. A group of five BALB/c mice orally infected with 10⁹ .sub.χ 3622 cells remained healthy and was totally unaffected (Table 3). Furthermore, these mice gained high-level immunity to oral challenge with 10⁸ parental .sub.χ 3339 cells (Table 3).

A series of strains was constructed to independently evaluate each of the phenotypic characteristics of .sub.χ 3622. The plasmid, pSD110, carrying the cloned crp⁺ gene and conferring ampicillin resistance (Schroeder and Dobrogosz, J. Bacteriol. 167:616-622 (1986)), was used to complement the Δcrp mutation in the chromosome. An L broth culture of .sub.χ 3622 was transduced with P22HTint propagated on S. typhimurium .sub.χ 3670, which contains the plasmid pSD110. Selection was made on MacConkey agar+1% maltose+100 μg ampicillin/ml. After 26 h, an ampicillin-resistant, Mal⁺ colony was picked and purified on MacConkey agar+1% maltose agar+100 μg ampicillin/ml and designated .sub.χ 3706. .sub.χ 3706 was administered perorally to mice and reisolated from the spleen. The animal-passaged strain was designated .sub.χ 3737. Two other crp mutants, .sub.χ 3605(crp-773::Tn10) and .sub.χ 3623 (Δcrp-11) that do not confer the Arg⁻ or Cys⁻ auxotrophic traits were also complemented with the pSD110 plasmid by transduction and designated .sub.χ 3731 and .sub.χ 3774, respectively. S. typhimurium strains independently carrying cysG and arg mutations were constructed and designated .sub.χ 3910 (cysG::Tn10), .sub.χ 4063 and .sub.χ 4071 (arg::Tn10).

Two other highly pathogenic S. typhimurium strains were selected for attenuation by introduction of the Δcrp-10 mutation. .sub.χ 3761 (UK-1) and 798 are virulent, invasive strains isolated from a moribund horse and pig, respectively, with LD₅₀ s in mice of approximately 1×10⁵ CFU. Transduction of Δcrp-10 with the linked transposon zhc-1431::Tn10 was facilitated by first making a high-titer bacteriophage P22HTint lysate on the S. typhimurium strain .sub.χ 3712 (see Table 1.B.). The phage lysate was then used to transduce the genetic traits into the wild-type recipient strains .sub.χ 3761 and 798. Tetracycline-resistant colonies were selected and screened for the Mal⁻, Arg⁻ and Cys⁻ phenotypes and the resulting 798 derivative designated .sub.χ 4246 and the .sub.χ 3761 (UK-1) derivative designated .sub.χ 4248 (Table 1).

The crp mutation was complemented by introducing pSD110, carrying the crp⁺ wild-type allele, into .sub.χ 4246 and .sub.χ 4248. L broth grown cultures of .sub.χ 4246 and .sub.χ 4248 were transduced with P22HTint propagated on S. typhimurium .sub.χ 3670, which contains the plasmid pSD110 (Table 1). Selection was made on MacConkey agar+1% maltose+100 μg ampicillin/ml+12.5 μg tetracycline/ml. After 26 h, an ampicillin, Mal⁺ colony of each strain was picked and purified on the same medium and designated .sub.χ 4247 (798) and .sub.χ 4262 (UK-1) which both have the genotype pSD110⁺ /Δcrp-10 zhc-1431::Tn10.

Virulence of the S. typhimurium .sub.χ 3622, .sub.χ 3731, .sub.χ 3737, .sub.χ 3774, .sub.χ 3910, .sub.χ 4063 and .sub.χ4071. Table 6 presents data on morbidity and mortality of mice infected perorally with the S. typhimurium strains .sub.χ 3622, .sub.χ 3731, .sub.χ 3737, .sub.χ 3774, .sub.χ 3910, .sub.χ 4063 and .sub.χ 4071. Strain .sub.χ 3737 was completely avirulent for mice that received 10⁴ times the LD₅₀ dose for the wild-type .sub.χ 3339 parent strain. Mice never appeared ill throughout the 30-day observation period. As a control for this experiment, the crp-773::Tn10 mutation in .sub.χ 3605 was complemented by pSD110 to the wild-type Crp⁺ phenotype (.sub.χ 3731) and mice were infected and died. Doses around 1×10⁵ CFU killed 4 of 5 mice p.o. inoculated with .sub.χ 3731 and .sub.χ 3774 (pSD110⁺ /ΔCrp-11). To test the virulence of strains with the Cys⁻ and Arg⁻ phenotypes independently, strains .sub.χ 3910 (cysG::Tn10), .sub.χ 4063 (arg::Tn10) and .sub.χ 4071 (arg::Tn10) were p.o. administered to BALB/c mice. .sub.χ 3910, .sub.χ 4063 and .sub.χ 4071 killed mice when similar or lower doses were p.o. administered. Therefore, the avirulence associated with the Δ[crp-cysG]-10 mutation was not solely due to deletion of the crp gene and was not conferred by deletion of either the argD or cysG loci. Rather, another gene necessary for S. typhimurium virulence must be localized to the region of chromosome near the crp gene.

                                      TABLE 6                                      __________________________________________________________________________     Virulence of S. typhimurium SL1344 Δ[crp-cysG]-10,                       Crp.sup.+ /crp;;Tn10 and Crp.sup.+ /Δ[crp-cysG]-10, arg;;Tn10,           cysG::Tn10                                                                     mutants in BALB/c mice 30 days after peroral inoculation                       Strain                                                                              Relevant  Inoculating                                                                          Survival                                                                             Mean day                                            number                                                                              genotype  dose (CFU)                                                                           live/total                                                                           of death.sup.a                                                                       Health.sup.b                                  __________________________________________________________________________     χ3339                                                                           wild-type 6 × 10.sup.4                                                                   2/5   7     scruffy                                       χ3622                                                                            ##STR119##                                                                              6 × 10.sup.8                                                                   5/5   --    healthy                                       χ3731                                                                           pSD110.sup.+                                                                             1 × 10.sup.5                                                                   1/5   9     scruffy                                             ##STR120##                                                               χ3737                                                                           pSD110+   5 × 10.sup.8                                                                   5/5   --    healthy                                             ##STR121##                                                               χ3774                                                                            ##STR122##                                                                              3 × 10.sup.4                                                                   3/5   12    scruffy                                       χ3910                                                                            ##STR123##                                                                              1 × 10.sup.7                                                                   0/2   12    scruffy                                       χ4063                                                                            ##STR124##                                                                              1 × 10.sup.9                                                                   0/2   8     scruffy                                       χ4071                                                                            ##STR125##                                                                              1 × 10.sup.9                                                                   0/2   9     scruffy                                       __________________________________________________________________________      .sup.a of animals that died                                                    .sup.b healthy-no noticeable signs of disease; moderately ill;                 scruffynoticeably ill.                                                   

Effectiveness of immunization with .sub.χ 3622, .sub.χ 3737, .sub.χ 4247 and .sub.χ 4262. Data on the ability of .sub.χ 3622, .sub.χ 3737, .sub.χ 4247 and .sub.χ 4262 to induce immunity to subsequent p.o. or i.p. challenge with 10⁴ times the LD₅₀ doses of fully virulent wild-type S. typhimurium cells are presented in Table 7. All mice given excessive doses of the wild-type parent strain never appeared ill throughout the 30-day duration of the experiment. Therefore the Δ[crp-cysG]-10 mutation deletes at least two genes both of which render S. typhimurium completely avirulent and highly immunogenic.

                                      TABLE 7                                      __________________________________________________________________________     Effectiveness of immunization with avirulent S. typhimurium                    Δ[crp-cysG]-10                                                           mutants in protecting against challenge with wild-type virulent parent         strains                                                                                      Dose (CFU)     Dose (CFU) of                                     Strain                                                                              Relevant of immunizing                                                                          Route of                                                                              wild-type                                                                              Survival                                  number                                                                              genotype strain  immunization                                                                          strain  live/total                                __________________________________________________________________________     χ3622                                                                            ##STR126##                                                                             6.2 × 10.sup.8                                                                   PO     3.6 × 10.sup.8                                                                   5/5                                                     1.5 × 10.sup.9                                                                   PO     3.2 × 10.sup.8                                                                   5/5                                                     4.2 × 10.sup.8                                                                   PO     8.8 × 10.sup.8                                                                   5/5                                                     9.0 × 10.sup.6                                                                   IP     1.4 × 10.sup.4                                                                   2/2                                                     9.0 × 10.sup.4                                                                   IP     1.4 × 10.sup.4                                                                   3/3                                                     9.0 × 10.sup.2                                                                   IP     1.4 × 10.sup.4                                                                   3/3                                       χ3737                                                                           pSD110.sup.+                                                                            5.8 × 10.sup.8                                                                   PO     8.4 × 10.sup.8                                                                   5/5                                             ##STR127##                                                               χ3955                                                                           pSD110.sup.+                                                                            6.8 × 10.sup.8                                                                   PO     8.4 × 10.sup.8                                                                   2/2                                             ##STR128##                                                               χ4247                                                                           pSD110.sup.+                                                                            2.0 × 10.sup.9                                                                   PO     9.8 × 10.sup.8                                                                   2/2                                             ##STR129##                                                               χ4262                                                                           pSD110.sup.+                                                                            1.5 × 10.sup.9                                                                   PO     5.4 × 10.sup.8                                                                   3/3                                             ##STR130##                                                               __________________________________________________________________________

Isolation of S. typhimurium strain with the Δcrp-14 mutation. Since an imprecise excision event of crp-773::Tn10 generated the deletion of genes extending from argD through cysG, another strategy was designed to locate the position of the gene conferring avirulence in the region adjacent to crp. Twenty independent deletion mutants of .sub.χ 3910 (cysG::Tn10) were selected on fusaric acid-containing medium and screened for tetracycline-sensitivity and maltose-negative phenotype. One of twenty fusaric acid-resistant derivative of .sub.χ 3910 had the genotype Δ[crp-cysG]-14 and conferred auxotrophy for histidine and cysteine, but not arginine. This strain, designated .sub.χ 3931, was transduced with a P22HTint lysate grown on .sub.χ 3670 to introduce pSD110 carrying the wild-type crp⁺ gene. An ampicillin-resistant, maltose-positive transductant was picked and purified on the same medium and the resulting strain was designated .sub.χ 3955.

Virulence of S. typhimurium pSD110⁺ /Δ[crp-cysG]-14 .sub.χ 3955. Table 7 shows morbidity and mortality of mice infected perorally with S. typhimurium .sub.χ 3955. Strain .sub.χ 3955 was completely avirulent for mice that received approximately 10⁹ CFU. Mice never appeared ill throughout the 30-day period.

Effectiveness of immunization with .sub.χ 3955. Table 7 shows the ability of .sub.χ 3955 to induce immunity to subsequent p.o. challenge with 10⁴ times the LD₅₀ dose of fully virulent wild-type S. typhimurium cells. Mice given excessive doses of the parent strain never appeared ill throughout the 30-day duration of the experiment.

Colonization of intestinal tract, GALT and spleen by .sub.χ 3622(Δ[crp-cysG]-10) and .sub.χ 3737 (pSD110⁺ Δ[crp-cysG]-10) relative to-the wild-type strain .sub.χ 3339. S. typhimurium .sub.χ 3622 and .sub.χ 3737 were grown and prepared for oral inoculation of 8-week-old female BALB/c mice as described in Example 1. Animals were sacrificed 1, 3, 5 and 7 days after p.o. inoculation with 9.4×10⁸ CFU (.sub.χ 3622), 1.2×10⁹ CFU (.sub.χ 3737) or 1.1×10⁹ CFU (.sub.χ 3339). Three mice per group were randomly selected, euthanized and tissue samples collected. The spleen, Peyer's patches, a 10-cm section of the ileum and the small intestinal contents from each mouse were placed in polypropylene tubes with BSG, homogenized with a Brinkmann tissue homogenizer and placed on ice. Undiluted or diluted samples (100 μl ) were plated directly on MacConkey agar+1% lactose+50 μg streptomycin/ml (.sub.χ 3339 and .sub.χ 3737) and MacConkey agar+1% maltose+50 μg streptomycin/ml (.sub.χ 3622) and the plates were incubated for 26 h 37° C. Titers in the perspective tissues were determined for each time period and the geometric mean calculated for 3 mice per group at each time of sampling.

The results of this analysis are presented in FIG. 1. It is evident that the additional attenuating mutation in .sub.χ 3622 and which is still manifested in the Crp⁺ (pSD110⁺) derivative .sub.χ 3737 very much diminishes the ability to effectively colonize deep tissues. The responsible gene which is deleted by the Δ[crp-cysG]-10 mutation has therefore been designated cdt. The Cdt⁻ phenotype of .sub.χ 3622 and .sub.χ 3737 is also manifested by the absence of any splenomegaly which is observed following p.o. inoculation of mice with S. typhimurium .sub.χ 3623 which has the Δcrp-11 mutation or with various other strains with combined Δcrp and Δcya mutations (Curtiss and Kelly, 1987). Strain .sub.χ 3737 grew more rapidly than .sub.χ 3622. The additional attenuating mutation in .sub.χ 3622 does not decrease growth rate as does the crp mutation.

Based on isolation and analysis of deletion mutations for phenotypes conferred, the order of genes in the S. typhimurium chromosome is inferred to be argD crp cdt cysG.

It is evident that inclusion of the Δ[crp-cysG]-10 or Δ[crp-cysG]-14 mutations which are also Δcdt mutations would enhance the safety of live attenuated Salmonella vaccine strains while not diminishing their immunogenicity. This might be particularly important for host-adapted invasive Salmonella species such as S. typhi, S. paratyphi A (S. schottmuelleri), S. paratyphi B (S. hirshfeldii), S. paratyphi C (all infect humans), S. choleraesuis (infects swine), S. dublin (infects cattle), S. gallinarum, and S. pullorum (both infect poultry), as well as non-host specific, invasive Salmonella species such as S. typhimurium and S. enteritidis.

Example 4

This example describes the construction of avirulent microbes by the introduction of deletion mutations affecting cAMP synthesis and utilization and an adjacent gene which also governs virulence of Salmonella by affecting colonization of deep tissues and the characterization of strains with two deletion mutations for stability of phenotype, complete avirulence and high immunogenicity.

Bacterial strains. The Escherichia coli and Salmonella typhimurium strains used are listed in Table 1.A. and B. The maintenance and storage of these strains are as described in Example 1.

Media. Complex media for routine cultivation, enumeration and identification of bacteria are as described in Example 1.

Transduction and fusaric acid selection for loss of Tn10. The media and methods are as described in Example 1.

Construction of S. typhimurium strains with Δcya-12 and Δ[crp-cysG]-10 deletion mutations. The best vaccine strains in terms of efficacy are likely to result from the attenuation of highly virulent strains that display significant colonizing ability and invasiveness. The criteria for selection of these highly pathogenic S. typhimurium wild-type strains such as SL1344 (.sub.χ 3339), UK-1 (.sub.χ 3761) and 798 has been described in Example 2.

The wild-type, virulent S. typhimurium strains SL1344, 798 and UK-1 were genetically modified as described below, using classical genetic methods similar to those described in Curtiss and Kelly (1987). The strategy consists of mobilizing deletions of crp and cya genes that have been isolated and characterized in S. typhimurium SL1344 (as described in Example 1) by placing the transposon Tn10 (encoding tetracycline resistance) nearby the Δcya-12 or Δ[crp-cysG]-10 mutation and transducing the linked traits into the highly virulent S. typhimurium strains UK-1 .sub.χ 3761, 798 and SL1344 .sub.χ 3339 via P22HTint-mediated transduction with selection for tetracycline resistance and screening for a maltose-negative phenotype. The zhc-1431::Tn10 linked to Δ[crp-cysG]-10 and zid-62::Tn10 linked to Δcya-12 were used for this purpose. Neither insertion alone affects the virulence of S. typhimurium.

Transduction of the gene deletions with the linked transposon was facilitated by first making a high-titer bacteriophage P22HTint lysate on the S. typhimurium strain .sub.χ 3712 containing the Δ[crp-cysG]-10 and zhc-1431::Tn10 mutations and another lysate on the S. typhimurium strain .sub.χ 3711 containing the Δcya-12 and zid-62::Tn10 mutations. The resulting P22HTint lysates were then used to transduce the genetic traits into the wild-type recipient strains .sub.χ 3339, 798 and .sub.χ 3761.

P22HTint propagated on S. typhimurium .sub.χ 3712 (Δ[crp-cysG]-10 zhc-1431::Tn10) was used to transduce the virulent strains to tetracycline resistance with screening for Mal⁻. The phage-bacteria infection mixtures were incubated for 20 min at 37° C. before 100 μl samples were spread onto MacConkey agar (Difco Laboratories, Detroit, Mich.) containing 1% maltose (final concentration) supplemented with 12.5 μg tetracycline/ml. After approximately 26 h incubation at 37° C., tetracycline resistant Mal⁻ transductants were picked and purified onto the same medium. The resulting 798 derivative was designated .sub.χ 3777 and the UK-1 derivative was designated .sub.χ 3779. Strains .sub.χ 3712, .sub.χ 3777 and .sub.χ 3779 all have the genotype Δ[crp-cysG]-10 zhc-1431::Tn10 (Table 1.B.). .sub.χ 3777 and .sub.χ 3779 were grown in L broth+12.5 μg tetracycline/ml and each were diluted 1:10 into buffered saline with gelatin (BSG), 100 μl of each were spread onto fusaric acid-containing (FA) media (Maloy and Nunn, 1981) and the plates were incubated approximately 36 h at 37° C. Fusaric acid-resistant colonies of each strain were picked into 0.5 ml BSG and purified onto FA medium. Purified fusaric acid-resistant colonies were picked into L broth and grown at 37° C. to turbidity and checked for loss of Tn10 (tetracycline sensitivity), presence of complete LPS and auxotrophy. The new strains were designated .sub.χ 3784 (UK-1) and .sub.χ 3806 (798) which both have the genotype Δ[crp-cysG]-10) Δ[zhc-1431::Tn10]. .sub.χ 3622 (SL1344 Δ[crp-cysG]-10) was originally isolated as described in Example 1) (Table 1B).

Since the phenotype of Cya⁻ and Crp⁻ mutants are the same (Mal⁻, Stl⁻, Mtl⁻, etc.), the plasmid, pSD110, carrying the cloned crp⁺ gene and conferring ampicillin resistance (Schroeder and Dobrogosz, J. Bacteriol 167:616-622(1986)), was used to temporarily complement the Δcrp mutation in the chromosome enabling the identification of the Δcya mutation when introduced via transduction. L broth grown cultures of .sub.χ 3622, .sub.χ 3784 and .sub.χ 3806 were transduced with P22HTint propagated on S. typhimurium .sub.χ 3670, which contains the plasmid pSD110 (Table 1). Selection was made on MacConkey agar+1% maltose+100 μg ampicillin/ml. After 26 h, an ampicillin-resistant, Mal⁺ colony of each strain was picked and purified on MacConkey agar+1% maltose agar+100 μg ampicillin/ml and designated .sub.χ 3901 (798) and .sub.χ 3945 (UK-1) which both have the genotype Δ[crp-cysG]-10 Δ[zhc-1431::Tn10] pSD110⁺ and .sub.χ 3706 (SL1344) which has the genotype Δ[crp-cysG]-10 pSD110⁺.

Strains .sub.χ 3706, .sub.χ 3901 and .sub.χ 3945 were grown in L broth+100 μg ampicillin/ml and were each independently transduced with P22HTint propagated on .sub.χ 3711 to introduce the linked Δcya-12 and zid-62::Tn10 mutations. The transduction mixtures were plated on MacConkey agar+1% maltose+100 μg ampicillin/ml+12.5 μg tetracycline/ml. Ampicillin-resistant (pSD110⁺), tetracycline-resistant (zid-62::Tn10), Mal⁻ (Δcya) colonies were picked and purified on MacConkey agar+1% maltose+100 μg ampicillin/ml+12.5 μg tetracycline/ml. Purified colonies were picked into L broth, grown to turbidity and the strains checked for complete LPS and auxotrophy. The resulting strains were designated .sub.χ 3902 (798) and .sub.χ 3956 (UK-1) which both have the genotype Δ[crp-cysG]-10 Δ[zhc-1431::Tn10] pSD110⁺ Δcya-12 zid-62::Tn10 and .sub.χ 3722 (SL1344) which has the genotype Δ[crp-cysG]-10 pSD110⁺ Δcya-12 zid-62::Tn10. Cultures of .sub.χ 3722, .sub.χ 3902 and .sub.χ 3956 were grown in L broth+100 μg ampicillin/ml+12.5 μg tetracycline/ml to turbidity, diluted 1:10 into BSG, and 100 μl samples of each culture spread onto fusaric acid-containing media and incubated approximately 36 h at 37° C. Fusaric acid-resistant colonies of each strain were picked and purified onto FA medium. Purified FA-resistant colonies were picked into L broth, grown to turbidity and then checked for loss of Tn10 (tetracycline sensitivity), complete LPS and auxotrophy. The pSD110 plasmid was usually lost spontaneously from the strains during this process to result in ampicillin sensitivity, except for the SL1344 and UK-1 derivatives which involved two steps to eliminate pSD110. The final strains were designated .sub.χ 3958 (UK-1) and .sub.χ 4038 (798) which both have the genotype Δ[crp-cysG]-10 Δ[zhc-1431::Tn10] Δcya-12 Δ[zid-62::Tn10] and .sub.χ 3724 (SL1344) which has the genotype Δ[crp-cysG]-10 Δcya-12 Δ[zid-62::Tn10] (Table 1.B.).

Genotypic and phenotypic stability of avirulent mutants. Methods for determining stability of genetic traits are as described in Example 1. All genotypic and phenotypic traits due to the Δcya Δcrp mutations were completely stable except motility. Although synthesis of functional flagella and display of motility is dependent on wild-type cya and crp gene functions, a suppressor mutation in the cfs (constitutive flagellar synthesis) gene can easily be selected to cause flagella synthesis and motility to be independent of cya and crp gene functions. In S. typhimurium Δcya Δcrp strains, motile variants were readily selected during the strain construction process. Since immunity to flagellar antigens may be protective, motile variants of all vaccine strains were selected.

S. typhimurium group B O-antigen synthesis was confirmed by slide agglutination with antisera (Difco Laboratories, Detroit, Mich.) and by P22HTint bacteriophage sensitivity by the Luria soft agar overlay technique.

Fermentation of sugars and growth on various carbon sources of the double mutant strains were identical to strains with only Δcya or Δcrp as listed in Table 2. The phenotypes were as expected based on published reports of the requirement for cyclic AMP and the cyclic AMP receptor protein for catabolic activities.

At each step in the construction following selection of a fusaric acid-resistant tetracycline-sensitive derivative, an investigation as to whether tetracycline-resistant revertants/mutants could be recovered at frequencies higher than could be observed for the tetracycline-sensitive wild-type parental strain was conducted. In all cases, such tetracycline-resistant revertants/mutants were not observed.

Example 5

This Example describes the construction of avirulent microbes by the introduction of deletion mutations affecting cAMP synthesis and utilization and the characterization of strains with two deletion mutations for stability of phenotype and complete avirulence.

Bacterial strains. The Salmonella typhimurium and S. typhi strains used are listed in Table 1.B. and C. The maintenance and storage of these strains are as described in Example 1.

Media. Complex media for routine cultivation, enumeration and identification of bacteria are as described in Example 1.

Transduction and fusaric acid selection for loss of Tn10. The media and methods are as described in Example 1.

Genetic stability of avirulent mutants. Methods for determining stability of genetic traits are as described in Example 1.

Mice. Male CD-1 mice (18-20 g) (Charles River, Wilmington, Mass.) were used for all infectivity experiments. Animals were held for one week in a quarantined room prior to being used in experiments. Experimental mice were placed in Nalgene filter-covered cages with wire floors. Food and water were given ad libitum. The animal room was maintained at 22°-23° C. with a period of 12 h illumination.

Animal infectivity. The virulence of S. typhi strains was determined following intraperitoneal (i.p.) injection with hog gastric mucin. Bacteria for inoculation into mice were grown overnight as standing cultures at 37° C. in L broth. The cultures were diluted 1:50 into prewarmed L broth and aerated at 37° C. for approximately 4 h to an OD₆₀₀ of about 0.8-1.0. Suitable dilutions were plated on Penassay agar for titer determination and on MacConkey agar with 1% maltose to verify the Cya/crp phenotype.

Intraperitoneal inoculation of unfasted CD-1 mice was performed using a 26-gauge 3/8" needle to deliver 500 μl of S. typhi cells suspended in 5% (w/v) hog gastric mucin (Wilson lot #0347A001). The mucin suspension was prepared by heating at 56° C. for 1 h, boiling for 6 min and neutralizing to pH 7 prior to adding S. typhi cells. LD₅₀ values of the wild-type parents and virulence of the Δcrp-11 Δcya-12 derivatives were determined after recording morbidity and mortality data for 10 days.

Construction of S. typhi strains with cya and crp mutations. The wild-type, virulent S. typhi Ty2 (type E1), ISP1820 (type 46) and ISP2822 (type E1) strains were genetically modified as described below, using classical genetic methods similar to those described in Curtiss and Kelly (1987). ISP1820 and ISP2822 were recently isolated during a typhoid epidemic in Chile and are likely to be more invasive than the standard laboratory Ty2 strain of S. typhi. Their attenuation might therefore generate vaccine strains that would be more efficacious than those derived from Ty2. The construction strategy consists of mobilizing deletions of crp and cya genes that have been isolated and characterized in S. typhimurium SL1344 by placing the transposon Tn10 (encoding tetracycline resistance) nearby the Δcya or Δcrp mutation and transducing the linked traits into the highly virulent S. typhi Ty2, ISP1820 and ISP2822 strains via P22HTint-mediated transduction with selection for tetracycline resistance and screening for a maltose-negative phenotype. The zhc-1431::Tn10 linked to crp and zid-62::Tn10 linked to cya were used for this purpose. Neither insertion alone affects virulence of S. typhimurium.

Transduction of the gene deletions with the linked transposon was facilitated by first making a high-titer bacteriophage P22HTint lysate on the S. typhimurium strain .sub.χ 3773 containing the Δcrp-11 and zhc-1431::Tn10 mutations and another lysate on the S. typhimurium strain .sub.χ 3711 containing the Δcya-12 and zid-62::Tn10 mutations. The resulting P22HTint lysates were then used to infect at a multiplicity of infection of 10 to transduce the genetic traits into the recipient S. typhi Ty2, ISP1820 and ISP2822 strains.

P22HTint propagated on S. typhimurium .sub.χ 3773 (Δcrp-11 zhc-1431::Tn10) was used to transduce the virulent S. typhi Ty2, ISP1820 and ISP2822 strains to tetracycline resistance with screening for Mal⁻. The phage-bacteria infection mixtures were incubated for 20 min at 37° C. before 100 μl samples were spread onto MacConkey agar (Difco Laboratories, Detroit, Mich.) containing 1% maltose (final concentration) supplemented with 12.5 μg tetracycline/ml. After approximately 26 h incubation at 37° C., tetracycline-resistant Mal⁻ transductants were picked and purified onto the same medium. The resulting Ty2 derivative was designated .sub.χ 3853, the ISP1820 derivative designated .sub.χ 4298 and the ISP2822 derivative designated .sub.χ 3852. All of these strains have the genotype Δcrp-11 zhc-1431::Tn10 (Table 1.C.). Strains .sub.χ 3852, .sub.χ 3853 and .sub.χ 4298 were grown in L broth+12.5 μg tetracycline/ml and each were diluted 1:10 into buffered saline with gelatin (BSG), 100 μl of each were spread onto fusaric acid-containing (FA) media (Maloy and Nunn, 1981) and the plates were incubated approximately 36 h at 37° C. Fusaric acid-resistant colonies of each strain were picked into 0.5 ml BSG and purified onto FA medium. Purified fusaric acid-resistant colonies were picked into L broth and grown at 37° C. to turbidity and checked for loss of Tn10 (tetracycline sensitivity), presence of complete LPS and Vi antigen and auxotrophy for cysteine and tryptophan (two amino acids required by all the parent strains). The new strains were designated .sub.χ 3877 (ISP2822), .sub.χ 3878 (Ty2) and .sub.χ 4299 (ISP1820) which all have the genotype Δcrp-11 Δ[zhc-1431::Tn10] (Table 1.C.).

Since the phenotype of Cya⁻ and Crp⁻ mutants are the same (Mal⁻, Stl⁻, Mtl⁻, etc.), the plasmid, pSD110, carrying the cloned crp⁺ gene conferring ampicillin resistance (Schroeder and Dobrogosz, J. Bacteriol. 167:616-622 (1986)), was used to temporarily complement the Δcrp mutation in the chromosome enabling the identification of the Δcya mutation when introduced via transduction. L broth grown cultures of .sub.χ 3877, .sub.χ 3878 and .sub.χ 4299 were transduced with P22HTint propagated on S. typhimurium .sub.χ 3670, which contains the plasmid pSD110 (Table 1.B.). Selection was made on MacConkey agar+1% maltose+100 μg ampicillin/ml. After 26 h, an ampicillin-resistant, Mal⁺ colony of each strain was picked and purified on MacConkey agar+1% maltose agar+100 μg ampicillin/ml and designated .sub.χ 3879 (ISP2822), .sub.χ 3880 (Ty2) and .sub.χ 4300 (ISP1820) which all have the genotype Δcrp-11 Δ[zhc-1431::Tn10] pSD110⁺.

Strains .sub.χ 3879, .sub.χ 3880 and .sub.χ 4300 were grown in L broth+100 μg ampicillin/ml and were each independently transduced with P22HTint propagated on .sub.χ 3711 to introduce the linked Δcya-12 and zid-62::Tn10 mutations. The transduction mixtures were plated on MacConkey agar+1% maltose+100 μg ampicillin/ml+12.5 μg tetracycline/ml. Ampicillin-resistant (pSD110⁺), tetracycline-resistant (zid-62::Tn10), Mal⁻ (Δcya) colonies were picked and purified on MacConkey agar+1% maltose+100 μg ampicillin/ml+12.5 μg tetracycline/ml. Purified colonies were picked into L broth, grown to turbidity and the strains checked for complete LPS, Vi antigen and auxotrophy for cysteine and tryptophan. The resulting strains were designated .sub.χ 3921 (ISP2822), .sub.χ 3922 (Ty2) and .sub.χ 4316 (ISP1820) which all have the genotype Δcrp-11 Δ[zhc-1431::Tn10] pSD110⁺ Δcya-12 zid-62::Tn10 (Table 1.C.). Cultures of .sub.χ 3921, .sub.χ 3922 and .sub.χ 4316 were grown in L broth+100 μg ampicillin/ml+12.5 μg tetracycline/ml to turbidity, diluted 1:10 into BSG, and 100 ml samples of each culture spread onto fusaric-containing media and incubated approximately 36 h at 37° C. Fusaric acid-resistant colonies of each strain were picked and purified onto FA medium. Purified FA-resistant colonies were picked into L broth, grown to turbidity and then checked for loss of Tn10 (tetracycline sensitivity), complete LPS, Vi antigen and auxotrophy for cysteine and tryptophan. The pSD110 plasmid was usually spontaneously lost from the strains during this process to result in ampicillin sensitivity. The final strains were designated .sub.χ 3926 (ISP2822), .sub.χ 3927 (Ty2) and .sub.χ 4322 (ISP1820) which all have the genotype Δcrp-11 Δ[zhc-1431::Tn10] Δcya-12 Δ[zid-62::Tn10] (Table 1.C.). S. typhi Vi antigen synthesis was confirmed by slide agglutination with antisera to Vi (Difco Laboratories, Detroit, Mich.) and by ViII bacteriophage sensitivity by the Luria soft agar overlay technique. Synthesis of flagella is dependent on functional cya and crp genes. However, since flagella are a potentially important antigen, motile derivatives of Δcya Δcrp S. typhi strains, due to mutation in the cfs (constitutive flagellar synthesis) gene (Silverman and Simon, J. Bacteriol. 120:1196-1203 (1974), were selected in motility agar. .sub.χ 3926 and .sub.χ 3927 were isolated as flagellated and motile whereas strain .sub.χ 4323 was selected as a flagella-positive motile derivative of .sub.χ 4222.

Table 8 lists the phenotypic properties of all the mutant strains and their parents with regard to fermentation of sugars and growth on various carbon sources, LPS profile, Vi antigen and mean generation time. The phenotypes are as expected based on published reports of the requirement for cyclic AMP and the cyclic AMP receptor protein for catabolic activities.

                  TABLE 8                                                          ______________________________________                                         Fermentation and growth properties of S. typhi strains                                      Phenotype                                                                      χ3745                                                                            χ3926                                                                               χ3769                                                                              χ3927                                  ______________________________________                                         MacConkey Base Agar +                                                                         +       -        +     -                                        1% maltose                                                                     MacConkey Base Agar +                                                                         +       -        +     -                                        1% sorbitol                                                                    MacConkey Base Agar +                                                                         +       -        +     -                                        1% mannitol                                                                    MacConkey Base Agar +                                                                         +       -        +     -                                        1% melibiose                                                                   MacConkey Base Agar +                                                                         -       -        -     -                                        1% rhamnose                                                                    MacConkey Base Agar +                                                                         -       -        -     -                                        1% citrate                                                                     MacConkey Base Agar +                                                                         -       -        -     -                                        1% arabinose                                                                   MacConkey Base Agar +                                                                         +       +        +     -                                        1% mannose                                                                     MacConkey Base Agar +                                                                         +       -        +     -                                        1% zylose                                                                      MacConkey Base Agar +                                                                         +       +        +     +                                        1% glucose                                                                     Minimal agar + +       +        +     +                                        0.5% glucose                                                                   Minimal agar + +       -        +     -                                        0.5% sorbitol                                                                  Minimal agar + +       -        +     -                                        0.5% mannitol                                                                  Minimal agar + +       -        +     -                                        0.5% melibiose                                                                 Minimal agar + -       -        -     -                                        0.5% rhamnose                                                                  Minimal agar + -       -        -     -                                        0.5% citrate                                                                   Minimal agar + -       -        -     -                                        0.5% arabinose                                                                 Minimal agar (continued)                                                                      + 0.5% mannose                                                                              +       ++-                                          "            + 0.5% xylose                                                                               +       -+-                                        Triple Sugar Iron media -                                                                     +       -        +     -                                        H.sub.2 S production                                                           alkaline slant =                                                                              Lac.sup.-                                                                              Lac.sup.-                                                                               Lac.sup.-                                                                            Lac.sup.-                                               Glu.sup.+                                                                              Glu.sup.+                                                                               Glu.sup.+                                                                            Glu.sup.+                                               Suc.sup.-                                                                              Suc.sup.-                                                                               Suc.sup.-                                                                            Suc.sup.-                                Indole fermentation assay                                                                     -       -        -     -                                        Bacteriophage sensitivity.sup.2                                                ViII    S          S       S      S                                            Felix-0                                                                               S           S       S      S                                            P22HTint                                                                              S           S       S      S                                            P1L4   R           R       R      R                                            L      R           R       R      R                                            KB1    R           R       R      R                                            LPS profile by SDS-PAGE                                                                       com-    com-     com-  com-                                     (silver strain)                                                                               plete   plete    plete plete                                    Motility.sup.bd                                                                               +       +        +     +                                        Colicin(s) production                                                                         -       -        -     -                                        MGT.sup.c              21.5     26.2  24.337.1                                 Plasmid content                                                                               none    none     none  none                                     Auxotrophy     Cys.sup.-                                                                              Cys.sup.-                                                                               Cys.sup.-                                                                            Cys.sup.-                                               Tryp.sup.-                                                                             Tryp.sup.-                                                                              Tryp.sup.-                                                                           .sup.Tryp-                               MIC.sup.d                                                                      Tetracycline    4       4       <2    4                                        Streptomycin   64      64        16   8                                        ______________________________________                                          .sup.a phage sensitivity was assayed by soft agar overlay technique or by      transduction. S  sensitive; R = resistant.                                     .sup.b Motility determined by stabbing a loopful of a standing overnight       culture into media containing 1.0% casein, 0.5% NaCl.sub.2, 0.5% Difco         agar, 50 μg/mg triphenyltetrazolium chloride indicator agar; incubatio      at 37° C. and motility recorded at 24 and 48 h.                         .sup.c Mean Generation Time (min) = determined in Luria broth with             aeration (150 rpm New Brunswick platform shaker) at 37° C.              .sup.d minimal inhibitory concentrations (μg/ml) of antibiotics were        determined by streaking standing overnight cultures of each strain onto        agar containing defined concentrations of antibiotics.                   

Genetic stability of avirulent mutants. Strains to be orally administered as live vaccines must have complete stability with regard to their avirulence attributes. When 50-fold concentrated cultures and various dilutions (˜10⁹, 10⁷, 10⁵, 10³ CFU/plate) of the Δcya Δcrp S. typhi strains were plated on minimal agar media (supplemented with required amino acids) containing 0.5% maltose, melibiose, xylose, glycerol, or rhamnose that should not support their growth, revertants and mutants were not detected. One set of duplicate plates was UV-irradiated (5 joules/meter² /sec) and incubated at 37° C. in the dark. The other set of plates was incubated at 37° C. with illumination. Revertants and mutants were not detected after a 48 h growth period. An investigation was also conducted as to whether tetracycline-resistant revertants/mutants could be recovered at frequencies higher than could be observed for the parental strain. In all cases, such tetracycline-resistant revertants/mutants were not observed.

Virulence of mutant strains for mice. Table 9 contains data which show that mice survive infection with about 10⁴ times the LD₅₀ dose of either .sub.χ 3926 or .sub.χ 3927. The natural host for S. typhi is man. Therefore, hog gastric mucin is used as a virulence enhancer of S. typhi cells in mice, and thus maximizes the virulence of S. typhi vaccine candidates in this model system.

Example 6

This Example demonstrates the construction of an avirulent microbe by the introduction of deletion mutations affecting cAMP synthesis and utilization and an adjacent gene which governs virulence of Salmonella by affecting colonization of deep tissues.

Bacterial strains. The Salmonella typhimurium and S. typhi strains used are listed in Table 1.B. and C. The maintenance and storage of these strains are as described in Example 1.

Media. Complex media for routine cultivation, enumeration and identification of bacteria are as described in Example 1.

Transduction and fusaric acid selection for loss of Tn10. The media and methods are as described in Example 1.

Genetic stability of avirulent mutants. Methods for determining stability of genetic traits are as described in Example 1.

Construction of S. typhi strains with Δcya-12 and Δ[crp-cysG]-10 mutations. S. typhi is highly invasive for humans. Although S. typhi strains with the Δcya-12 and Δcrp-11 mutations appear to be avirulent, it would seem prudent to consider adding an additional attenuating mutation to further enhance safety without compromising immunogenicity. The properties of the Δ[crp-cysG]-10 mutation in S. typhimurium strains (Examples 1, 3, and 4) justify its use to render S. typhi avirulent and immunogenic. This mutation also deletes the cdt gene governing colonization of deep tissues by Salmonella typhimurium without significantly diminishing colonization of the intestinal tract and GALT.

The wild-type, virulent Ty2 (type E1), ISP1820 (type 46) and ISP2822 (type E1) strains were genetically modified as described below, using classical genetic methods similar to those described in Curtiss and Kelly (1987). ISP1820 and ISP2822 were recently isolated during a typhoid epidemic in Chile and are likely to be more invasive than the standard laboratory Ty2 strain of S. typhi. Their attenuation might therefore generate vaccine strains that could be more efficacious than those derived from Ty2. The construction strategy consists of mobilizing deletions of crp and cya genes that have been isolated and characterized in S. typhimurium SL1344 (as described in Example 1) by placing the transposon Tn10 (encoding tetracycline resistance) nearby the Δcya or Δcrp mutation and transducing the linked traits into S. typhi Ty2 and the highly virulent S. typhi ISP1820 and ISP2822 strains via P22HTint-mediated transduction with selection for tetracycline resistance and screening for a maltose-negative phenotype. The zhc-1431::Tn10 linked to crp and zid-62::Tn10 linked to cya were used for this purpose. Neither insertion alone affects virulence of S. typhimurium.

Transduction of the gene deletions with the linked transposon was facilitated by first making a high-titer bacteriophage P22HTint lysate on the S. typhimurium strain .sub.χ 3712 containing the Δ[crp-cysG]-10 and zhc-1431::Tn10 mutations and another lysate on the S. typhimurium strain .sub.χ 3711 containing the Δcya-12 and zid-62::Tn10 mutations. The resulting P22HTint lysates were then used to transduce the genetic traits into the recipient S. typhi Ty2, ISP1820 and ISP2822 strains.

P22HTint propagated on S. typhimurium .sub.χ 3712 (Δ[crp-cysG]-10 zhc-1431::Tn10) was used to transduce the virulent S. typhi Ty2, ISP1820 and ISP2822 strains to tetracycline resistance with screening for Mal⁻. The phage-bacteria infection mixtures were incubated for 20 min at 37° C. before 100 μl samples were spread onto MacConkey agar (Difco Laboratories, Detroit, Mich.) containing 1% maltose (final concentration) supplemented with 12.5 μg tetracycline/ml. After approximately 26 h incubation at 37° C., tetracycline-resistant Mal⁻ transductants were picked and purified onto the same medium. The resulting ISP2822 derivative was designated .sub.χ 3791, the Ty2 derivative was designated .sub.χ 3792, and the ISP1820 derivative was designated .sub.χ 4324. All of these strains have the genotype Δ[crp-cysG]-10 zhc-1431::Tn10 and were auxotrophic for cysteine, tryptophan and arginine (Table 1.C.). Strains .sub.χ 3791, .sub.χ 3792 and .sub.χ 4324 were grown in L broth+12.5 μg tetracycline/ml. Each culture was diluted 1:10 into buffered saline with gelatin (BSG), 100 μl of each was spread onto fusaric acid-containing (FA) media (Maloy and Nunn, 1981) and the plates incubated approximately 36 h at 37° C. Fusaric acid-resistant colonies of each strain were picked into 0.5 ml BSG and purified onto FA medium. Purified fusaric acid-resistant colonies were picked into L broth and grown at 37° C. to turbidity and checked for loss of Tn10 (tetracycline sensitivity), presence of complete LPS and Vi antigen and auxotrophy for cysteine, arginine and tryptophan. The new strains were designated .sub.χ 3802 (ISP2822), .sub.χ 3803 (Ty2) and .sub.χ 4325 (ISP1820) which all have the genotype Δ[crp-cysG]-10 Δ[zhc-1431::Tn10] (Table 1.C.).

Since the phenotype of Cya⁻ and Crp⁻ mutants are the same (Mal⁻, Stl⁻, Mtl⁻, etc.), the plasmid, pSD110, carrying the cloned crp⁺ gene and conferring ampicillin resistance (Schroeder and Dobrogosz, J. Bacteriol. 167:616-622 (1986)), was used to temporarily complement the Δcrp mutation in the chromosome enabling the identification of the Δcya mutation when introduced via transduction. L broth grown cultures of .sub.χ 3802, .sub.χ 3803 and .sub.χ 4325 were transduced with P22HTint propagated on S. typhimurium .sub.χ 3670, which contains the plasmid pSD110 (Table 1.B.). Selection was made on MacConkey agar+1% maltose+100 μg ampicillin/ml. After 26 h, an ampicillin-resistant, Mal⁺ colony of each strain was picked and purified on MacConkey agar+1% maltose agar+100 μg ampicillin/ml and designated .sub.χ 3824 (Ty2), .sub.χ 3845 (ISP2822) and .sub.χ 4331 (ISP1820) which all have the genotype Δ[crp-cysG]-10 Δ[zhc-1431::Tn10] pSD110⁺.

Strains .sub.χ 3824, .sub.χ 3845, and .sub.χ 4331 were grown in L broth+100 μg ampicillin/ml and were each independently transduced with P22HTint propagated on .sub.χ 3711 to introduce the linked Δcya-12 and zid-62::Tn10 mutations. The transduction mixtures were plated on MacConkey agar+1% maltose+100 μg ampicillin/ml+12.5 μg tetracycline/ml. Ampicillin-resistant (pSD110⁺), tetracycline-resistant (zid-62::Tn10), Mal⁻ (Δcya) colonies were picked and purified on MacConkey agar+1% maltose+100 μg ampicillin/ml+12.5 μg tetracycline/ml. Purified colonies were picked into L broth, grown to turbidity and the strains checked for complete LPS, Vi antigen and auxotrophy for cysteine and tryptophan. The resulting strains were designated .sub.χ 3919 (Ty2), .sub.χ 3920 (ISP2822) and .sub.χ 4340 (ISP1820) which all have the genotype Δ[crp-cysG]-10 Δ[zhc-1431::Tn10] pSD 110⁺ Δcya-12 zid-62::Tn10. Cultures of .sub.χ 3919, .sub.χ 3920 and .sub.χ 4340 were grown in L broth+100 μg ampicillin/ml+12.5 μg tetracycline/ml to turbidity, diluted 1:10 into BSG, and 100 μl samples of each culture spread onto fusaric acid-containing media and incubated approximately 36 h at 37° C. Fusaric acid-resistant colonies of each strain were picked and purified onto FA medium. Purified FA-resistant colonies were picked into L broth, grown to turbidity and then checked for loss of Tn10 (tetracycline sensitivity), complete LPS, Vi antigen and auxotrophy for cysteine, arginine and tryptophan. The pSD110 plasmid was usually spontaneously lost from the strains during this process to result in ampicillin sensitivity. The final strains were designated .sub.χ 3924 (Ty2), .sub.χ 3925 (ISP2822) and .sub.χ 4345 (ISP1820) which all have the genotype Δ[crp-cysG]-10 Δ[zhc-1431::Tn10] Δcya-12 Δ[zid-62::Tn10] (Table 1.C.). S. typhi Vi antigen synthesis was confirmed by slide agglutination with antisera to Vi (Difco Laboratories, Detroit, Mich.) and by ViII bacteriophage sensitivity by the Luria soft agar overlay technique. Synthesis of flagella is dependent on functional cya and crp genes. However, since flagella are a potentially important antigen, motile derivatives of Δcya Δcrp S. typhi strains, due to mutation in the cfs (constitutive flagellar synthesis) gene (Silverman and Simon, J. Bacteriol. 120:1196-1203 (1974), were selected in motility agar. Strains .sub.χ 3940 (ISP2822), .sub.χ 4073 (Ty2) and .sub.χ 4346 (ISP1820) were selected as flagella-positive motile derivatives of .sub.χ 3925, .sub.χ 3924 and .sub.χ 4345, respectively.

Fermentation of sugars and growth on various carbon sources of the Δ[crp-cysG]-10 mutant strains were the same as observed for the Δcrp-11 mutant strains. The phenotypes are as expected based on published reports of the requirement for cyclic AMP and the cyclic AMP receptor protein for catabolic activities.

Genetic stability of avirulent mutants. Strains to be orally administered as live vaccines must have complete stability with regard to their avirulence attributes. When 50-fold concentrated cultures and various dilutions (˜10⁹, 10⁷, 10⁵, 10³ CFU/plate) of the Δcya Δcrp S. typhi strains were plated on minimal agar media (supplemented with required amino acids) containing 0.5% maltose, melibiose, xylose, glycerol, or rhamnose that should not support their growth, revertants and mutants were not detected. One set of duplicate plates was UV-irradiated (5 joules/meter² /sec) and incubated at 37° C. in the dark. The other set of plates was incubated at 37° C. with illumination. Revertants and mutants were not detected after a 48 h growth period. An investigation was also conducted as to whether tetracycline-resistant revertants/mutants could be recovered at frequencies higher than could be observed for the parental strain. In all cases, such tetracycline-resistant revertants/mutants were not observed.

Example 7

This Example describes the construction of recombinant avirulent S. typhi strains expressing foreign antigens for use as oral vaccines to immunize against various infectious diseases.

Bacterial strains. The E. coli, S. typhimurium and S. typhi strains used are listed in Table 1. The maintenance and storage of these strains are as described in Example 1.

Media. Complex media for routine cultivation, enumeration and identification of bacteria are as described in Example 1.

Transduction and fusaric acid selection for loss of Tn10. The media and methods are as described in Example 1.

Construction of S. typhi strains with ΔasdA1 mutation. The wild-type, virulent S. typhi Ty2 (type E1) was genetically modified as described below, using classical genetic methods similar to those described in Curtiss and Kelly (1987) and Nakayama, Kelly and Curtiss (1988). The construction of strain .sub.χ 3927 containing the Δcya-12 Δcrp-11 mutations was described in Example 5. The stable maintenance and high-level expression of cloned genes on recombinant plasmids in avirulent Salmonella strains is dependent upon use of a balanced-lethal host-vector system. For this, a chromosomal mutation of the asd gene encoding aspartate β-semialdehyde dehydrogenase is introduced into a Δcya Δcrp mutant to impose an obligate requirement for diaminopimelic acid (DAP) which is an essential constituent of the rigid layer of the bacterial cell wall and which is not synthesized in animals. The chromosomal Δasd mutation is then complemented by a plasmid cloning vector possessing the wild-type asd⁺ gene. Loss of the plasmid results in DAPless death and cell lysis. Such balanced-lethal host-vector combinations are stable for several weeks in the immunized animal host and elicit strong immune responses against the cloned gene product as well as against Salmonella.

The construction strategy consists of mobilizing the ΔasdA1 mutation that has been isolated and characterized in S. typhimurium LT2-Z (.sub.χ 3520) into a Δcya Δcrp S. typhi strain. This was accomplished by placing the transposon Tn10 (encoding tetracycline resistance) nearby the ΔasdA1 mutation and transducing the linked traits into the S. typhi Ty2 Δcya-12 Δcrp-11 strain .sub.χ 3927 via P22HTint transduction with selection for tetracycline resistance and screening for a diaminopimelic acid (DAP)-negative phenotype. The zhf-4::Tn10 linked to ΔasdA1 was used for this purpose.

Transduction of the gene deletion with the linked transposon was facilitated by first making a high-titer bacteriophage P22HTint lysate on S. typhimurium .sub.χ 3520 containing the ΔasdA1 and zhf-4::Tn10 mutations. The resulting P22HTint lysate was then used to infect and transduce the genetic traits into the recipient S. typhi Ty2 strain .sub.χ 3927 at a multiplicity of infection of 10.

The phage-bacteria infection mixture was incubated for 20 min at 37° C. before 100 μl samples were spread onto Penassay agar (Difco Laboratories, Detroit, Mich.) containing 50 μg DAP/ml and supplemented with 12.5 μg tetracycline/ml. After approximately 26 h incubation at 37° C. transductants were picked and purified on the same medium. A screening of five tetracycline-resistant colonies yields approximately four to five transductants that are also DAP-requiring. The resulting Ty2 derivative was designated .sub.χ 4296 and has the genotype Δcrp-11 Δ[zhc-1431::Tn10] Δcya-12 Δ[zid-62::Tn10] ΔasdA1 zhf-4104::Tn10. Strain .sub.χ 4296 was grown in L broth+50 μg DAP/ml+12.5 μg tetracycline/ml and was diluted 1:10 into buffered saline with gelatin (BSG), 100 μl was spread onto fusaric acid-containing (FA)+50 μg DAP/ml medium (Maloy and Nunn, 1981) and the plates were incubated approximately 36 h at 37° C. Fusaric acid-resistant colonies were picked into 0.5 ml BSG and purified onto FA+50 μg DAP/ml media. Purified fusaric acid-resistant colonies were picked into L broth+50 μg DAP/ml and grown at 37° C. to turbidity and checked for loss of Tn10 (tetracycline sensitivity), presence of complete LPS and Vi antigen and auxotrophy for cysteine, tryptophan, methionine, threonine and DAP on minimal media. The new strain was designated .sub.χ 4297 which has the genotype Δcrp-11 Δ[zhc-1431::Tn10] Δcya-12 Δ[zid-62::Tn10] ΔasdA1 Δ[zhf-4::Tn10].

Expression of a Mycobacterium leprae antigen in avirulent recombinant S. typhi. μgt11::Mycobacterium leprae clone L14 (also designated clone 7.8) was identified by immunological screening of a μgt11::M. lepraelibrary with pooled sera from 21 lepromatous (LL) leprosy patients (Sathish, Esser, Thole and Clark-Curtiss, Infect. Immun. 58:1327-1336 (1990)). Clone L14 specifies two proteins of approximately 158 and 153 kDa, both of which react very strongly with antibodies in the pooled LL patients' sera (Sathish et al., 1990). These proteins also react with antibodies in 14 out of the 21 LL patients' sera when the sera were tested individually (Clark-Curtiss, Thole, Sathish, Bosecker, Sela, de Carvalho and Esser, Res. in Microbiology, in press).

The 1.0 kb M. leprae insert DNA fragment was removed from μgt11 clone L14 by digestion of the recombinant phage DNA with EcoRI, followed by separation of the digestion fragments by agarose gel electrophoresis. The M. leprae fragment was purified from the gel and cloned into the EcoRI site of the Asd⁺ vector pYA292 (Galan, Nakayama and Curtiss, Gene, in press). Two kinds of recombinant plasmids were generated: pYA1077, in which the M. leprae insert DNA was cloned into pYA292 in the same orientation relative to the trc promoter as it was in μgt11 relative to the lacZ promoter, and pYA1078, in which the M. leprae fragment was cloned in the opposite orientation relative to the trc promoter. A partial restriction map of pYA1077 is presented in FIG. 2. Both recombinant plasmids were transformed into Escherichia coli K-12 strain .sub.χ 6060 and S. typhimurium strain .sub.χ 3730 and the proteins specified by the transformants were analyzed by Western blotting. Clone pYA1077 specifies a single fusion protein of approximately 30 kDa, which reacts strongly with antibodies in the pooled LL patients' sera. Clone pYA1078 does not specify any protein that reacts with the patients' sera.

Bacteriophage P22HTint lysates were prepared on S. typhimurium .sub.χ 3730+pYA1077 and .sub.χ 3730 +pYA1078; these lysates were used to transduce S. typhi .sub.χ 4297. Western blot analysis of the proteins produced by three randomly chosen transductants of .sub.χ 4297 with pYA1077 showed that each transductant specified a protein of 30 kDa that reacted with the pooled LL patients' sera whereas three independent .sub.χ 4297 transductants harboring pYA1078 did not specify an immunologically reactive protein (FIG. 3). The S. typhi strain .sub.χ 4297 with the pYA1077 recombinant vector has potential utility to immunize humans to protect against typhoid fever and leprosy. Efficacy of such vaccines will be dependent upon identifying one to several M. leprae antigens that would elicit protective immune responses and having them specified by cloned genes in an Asd⁺ vector in the S. typhi Δcya Δcrp Δasd strain .sub.χ 4297 which could then be used in human immunization trials.

Example 8

This example provides a procedure for testing the safety, immunogenicity, and efficacy of live oral vaccines comprised of Δcya Δcrp mutants of S. typhi. The strains tested are Δcya Δcrp derivatives of Ty2, ISP1820 and ISP2822.

The Individuals Studied. The individuals studied are volunteers who are healthy adult humans aged 18-39 years. The prospective volunteers are screened before the study. The screening procedure includes:

1. medical history

2. physical examination

3. electrocardiogram

4. urinalysis

5. complete blood count

6. blood chemistries (BUN, creatinine, fasting blood glucose

7. Serum Na⁺, Cl⁻, K⁺, HCO₃ ⁻

8. VDRL

9. Hepatitis B surface antigen

10. HIV antibody by ELISA

11. Pregnancy test (females)

12. Liver function tests (SPGT)

13. Psychological examination and interviews.

The Volunteers to participate in the study are selected on the basis of general good health and have:

1. no clinically significant history of gall bladder disease, immunodeficiency, cardiovascular disease, respiratory disease, endocrine disorder, liver disease including a history of hepatitis, renal and bladder disease, enlarged prostate, glaucoma, gastrointestinal disease, disorder of reticuloendothelial system, neurologic illness, psychiatric disorder requiring hospitalization, drug or alcohol abuse;

2. normal and regular bowel habits falling within the limits defined for a normal population: at least 3 stools per week and less than 3 stools per day without frequent use of laxatives or antidiarrheal agents;

3. absence of allergy to amoxicillin or ciprofloxacin;

4. no history of any antibiotic therapy during the 7 days before vaccination;

5. a negative pregnancy test (females);

6. a negative HIV antibody test.

The Volunteers are admitted to an Isolation Ward, and informed, witnessed, written consent is obtained.

Study design. Groups of 22 volunteers are studied. Baseline blood and intestinal fluid specimens are collected. After a two-day period of acclimitization on the ward, the fasting volunteers are randomly allowed to ingest with bicarbonate buffer a single oral dose containing 5×10⁵ of either the Δcya Δcrp derivative of Ty2, ISP1820 or ISP2822. The volunteers are observed for the next 15 days for adverse reactions (fever, malaise, chills, vomiting, diarrhea)(the usual incubation period of typhoid fever is 8-12 days). Serial blood and stool cultures are obtained. In addition, any volunteer who has a temperature elevation to 100.8° F. has blood samples drawn at the time the observation is made; if the temperature remains elevated at this level for 12 hours, therapy is initiated with oral amoxicillin (1.0 gram every 6 h) and oral ciprofloxacin (750 mg every 12 h for 10 days). Duodenal fluid cultures are also obtained during the period of observation on days 7, 10, and 13.

Animal tests. The LD₅₀ s for the parent strains and attenuated derivatives in mice by intraperitoneal inoculation with hog gastric mucin as adjuvant are also determined.

Preparation of the vaccine inocula. Stock cultures of the S. typhi candidate vaccine strains are stored as a cell suspension in trypticase soy broth (TSB), supplemented with 15% glycerol, at -70° C. until needed. To make an inoculum of each strain, the suspension is thawed and plated onto sheep red blood cell agar (5% srbc in TSA), two days before challenge. After incubation at 37° C. overnight, about 20-30 typical colonies are picked and suspended in saline. This suspension is inoculated onto trypticase soy agar plates, appropriately supplemented, and the plates incubated overnight at 37° C. In preparation for orally vaccinating the volunteers, growth on these plates is harvested with approximately 3 ml sterile normal saline per plate. The resulting suspension is standardized turbidimetrically. Dilutions are made in saline to approximate the concentration of Salmonella required. The vaccine inoculum is transported to She isolation ward on ice. Microscopic examination and slide agglutination with S. typhi O and H antisera are performed before use. Replica spread plate quantitative cultures are made of the inocula before and after vaccination to confirm viability and inoculum size.

Inoculation of Volunteers. The vaccine is administered by the oral route with NaHCO₃. Volunteers are NPO for 90 minutes before vaccination. Two grams of NaHCO₃ are dissolved in 5 ounces of distilled water. Volunteers drink 4 ounces of the bicarbonate water; one minute later the volunteers ingest the vaccine suspended in the remaining 1 ounce of bicarbonate water. Volunteers take no food or water for 90 minutes after inoculation.

Procedures for Specimen Collection.

Stool specimens. A record is kept of the number, consistency, and description of all stools passed by volunteers. A specimen of every stool (or rectal swab if stool is not passed) is collected for culture. The volume of the specimen is measured. Stools are graded on a five point system:

grade 1-firm stool (normal)

grade 2-soft stool (normal)

grade 3-thick liquid (abnormal)

grade 4-opaque watery (abnormal)

grade 5-rice water (abnormal).

Phlebotomy. Serum for antibody determinations is obtained before and 8, 21, 28, 60, and 180 days after vaccination. Heparinized blood for lymphocyte separations for antibody-secreting cell assays is collected on days 0, 4, 7, and 10. Mononuclear cells collected on days 0, 28, 60, and 180 days are used to assess lymphocyte proliferative responses to Salmonella and control antigens. Lastly mononuclear cells from days 0, 28, 60, and 180 are also used in the antibody-dependent cytotoxicity assay against S. typhi and control organisms. Blood (5 ml) is obtained for culture on days 3, 4, 7, 8, 10, 12, and 15 during the post-vaccination observation period to detect vaccine organisms. An additional specimen of serum and mononuclear cells are obtained 180 days after primary vaccination.

Jejunal fluid aspiration. Before oral vaccination and immediately before discharge (day 15), volunteers swallow polyvinyl chloride intestinal tubes to a distance of 130 cm from the mouth to collect intestinal fluid for measurement of local SIgA antibody. Ten mg of metoclopramide is given orally after ingestion of the tube to accelerate its passage from the stomach through the pylorus into the small intestine. Placement of the tubes in the jejunum is verified by distance (130 cm), color (yellow-green), and pH (6) of aspirated fluid. Approximately 100 ml of jejunal fluid is removed at each intubation.

Gelatin String Capsules. In order to determine rates of intestinal colonization with each vaccine strain, gelatin string capsules (Entero-Test) are ingested by volunteers three times during the period of hospitalization.

The volunteer is NPO from 6 A.M. A swallow of water is used to moisten the mouth and throat. The capsule, with a portion of the string pulled out, is swallowed with water while holding the loop of the nylon string. The line is secured to the face, and left in place for 4 hours. The volunteers are allowed to drink water ad lib, but are not allowed other food or beverages. After 4 hours, the line is withdraw, the distal section saturated with bile stained mucus is cut and placed in a sterile petri dish, which is labeled for identification. The strings are then cultured for microorganisms, using the same method as with the stool specimens.

Tonsillar Cultures. In order to detect possible invasion of tonsillar lymph tissue after vaccination, serial tonsillar cultures are obtained on days 3, 4, 7, 8, 10, 12, and 15.

Bacteriological Analysis. Stools, rectal swabs, and the distal 15 cm of bile-stained duodenal string from the ingested gelatin capsule is inoculated into selenite F enrichment broth. Tonsillar swabs are inoculated into GN broth. After overnight incubation at 37° C., subcultures are made onto Salmonella-Shigella agar and XLD agar, both appropriately supplemented for the auxotrophy of the vaccine strain. Suspicious colonies are transferred to supplemented triple sugar iron slants and confirmation made by agglutination with S. typhi Vi, O, and H antisera. These isolates are saved at -70° C. in glycerol for further analysis (e.g., for the presence of plasmids or for Southern blotting with specific gene probes for cloned genes).

Blood cultures (5 ml) are inoculated into 50 ml of supplemented brain heart infusion broth.

Immunological Analysis. Sera and jejunal fluid specimens are tested for IgA, IgM, and IgG antibodies to S. typhi O, H, and Vi antigens measured by ELISA, using the procedures described by Levine et al. (1987), J. Clin. Invest. 79:888-902. H antibody is also measured by Widal tube agglutination using S. virginia as antigen (S. virginia shares an identical flagellar antigen with S. typhi).

Peripheral blood mononuclear cells are collected and separated for studies of specific responses to Salmonella antigens. These include the following.

1. Antibody-secreting cells: trafficking lymphocytes which secrete IgG, IgA or IgM antibody against S. typhi O, Vi or H antigens are measured by the method of Kantele et al.

2. Replicating lymphocytes: peripheral blood mononuclear cells are mixed with heat-phenol-inactivated S. typhi, S. typhimurium, S. thompson, and E. coli to detect antigen-driven lymphocyte replication, as described in Levine et al., supra.

3. ADCC: plasma-mediated mononuclear cell inhibition of S. typhi is measured in an antibody dependent cellular cytotoxicity assay as described in Levine et al., supra.

Excretion of the Vaccine Strain. It is expected that excretion of the vaccine strain would cease within 1 week after a dose of vaccine. If excretion continues for 7 or more days, the volunteer who continues to excrete is given a dose of ciprofloxacin (750 mg every 12 hours). Negative cultures for ≧2 consecutive days are required for discharge.

Deposits of Strains. The following listed materials are on deposit under the terms of the Budapest Treaty, with the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Md. The accession number indicated was assigned after successful viability testing, and the requisite fees were paid. Access to said cultures will be available during pendency of the patent application to one determined by the Commissioner to be entitled thereto under 37 CFR 1.14 and 35 USC 122. All restriction on availability of said cultures to the public will be irrevocably removed upon the granting of a patent based upon the application. Moreover, the designated deposits will be maintained for a period of thirty (30) years from the date of deposit, or for five (5) years after the last request for the deposit, or for the enforceable life of the U.S. patent, whichever is longer. Should a culture become nonviable or be inadvertantly destroyed, or, in the case of plasmid-containing strains, lose its plasmid, it will be replaced with a viable culture(s) of the same taxonomic description. The deposited materials mentioned herein are intended for convenience only, and are not required to practice the present invention in view of the description herein, and in addition, these materials are incorporated herein by reference.

    ______________________________________                                         Strain       Deposit Date ATCC No.                                             ______________________________________                                         χ3958    November 2, 1990                                                                            55110                                                χ4323    November 2, 1990                                                                            55115                                                χ3926    November 2, 1990                                                                            55112                                                χ3927    November 2, 1990                                                                            55117                                                χ4297    November 2, 1990                                                                            55111                                                χ4346    November 2, 1990                                                                            55113                                                χ3940    November 2, 1990                                                                            55119                                                χ4073    November 2, 1990                                                                            55118                                                ISP2822      November 2, 1990                                                                            55114                                                ISP1820      November 2, 1990                                                                            55116                                                ______________________________________                                    

Commercial Utility

The strains provided herein are directly and indirectly suitable for the production of immunogenic compositions, including vaccines, to prevent diseases caused by S. typhi, and other enteric bacteria with which antibodies to S. typhi cross react. These strains are also useful as carrier microorganisms for the production of expression products encoded on recombinant genes in the bacterial cells. In addition, the strains which can be used with enhanced safety are useful for the production of antibodies, both monoclonal and polyclonal, against S. typhi, and against antigens which are expressed in the avirulent S. typhi. 

We claim:
 1. An immunogenic composition for the immunization of an individual comprising a live avirulent Salmonella having a mutation in a cdt gene said live avirulent Salmonella having the phenotype of failure to colonize deep tissue of Salmonella deposit strain ATCC no.
 55113. 2. An immunogenic composition for the immunization of an individual according to claim 1, wherein said avirulent Salmonella expresses a recombinant gene from an agent pathogenic to said individual, to produce an antigen which induces an immune response in said vertebrate against said pathogen.
 3. A method for stimulating the immune system of an individual to respond to an immunogenic antigen of Salmonella comprising administering to said individual an immunogenic composition comprising a live avirulent Salmonella having a mutation in a cdt gene said live avirulent Salmonella having the phenotype of failure to colonize deep tissue of Salmonella deposit strain ATCC no.
 55113. 4. A method for stimulating the immune system to respond to an immunogenic antigen of a pathogen comprising administering to said individual an immunogenic composition comprising a live avirulent Salmonella having a mutation in a cdt gene said live avirulent Salmonella having the phenotype of failure to colonize deep tissue of Salmonella deposit strain ATCC no.
 55113. 5. A biologically pure live avirulent strain of Salmonella said live avirulent Salmonella having the phenotype of failure to colonize deep tissue of Salmonella deposit strain ATCC no.
 55113. 6. The avirulent strain of Salmonella of claim 5, which expresses a recombinant gene from an agent pathogenic to said individual, to produce an antigen which induces an immune response in said vertebrate against said pathogen.
 7. A strain according to claim 6, wherein the Salmonella contains a chromosomal mutation which is lethal and which is balanced by a vector-borne gene which complements the lethal mutation to constitute a balanced lethal host vector system.
 8. A strain according to claim 6, wherein cells of the strain:a) lack a functioning native chromosomal gene encoding beta-aspartate semialdehyde dehydrogenase asd; b) have present an exogenously introduced gene encoding a functional Asd polypeptide which phenotypically complements the chromosomal asd mutation, but which cannot replace the defective chromosomal gene by recombination; and c) have a physical linkage between the recombinant genes encoding the functional Asd polypeptide and the immunogenic antigen, wherein the loss of the recombinant gene encoding the functional Asd polypeptide causes the cells to lyse when the cells are in an environment in which the lack of functional Asd causes the cells to lyse.
 9. A live biologically pure strain of S. typhi having a mutation in a cdt gene said live avirulent Salmonella having the phenotype of failure to colonize deep tissue of Salmonella deposit strain ATCC no.
 55113. 10. A vaccine for the immunization of an individual comprising:a pharmaceutically effective mount of a live avirulent Salmonella which has a diminished ability to colonize deep tissue of said individual as a result of a mutation in a cdt gene said live avirulent Salmonella having the phenotype of failure to colonize deep tissue of Salmonella deposit strain ATCC no.
 55113. 11. The vaccine of claim 10 wherein said avirulent Salmonella fails to colonize deep tissue of said individual. 2 